Things are never quite what they seem to be, you only have to look at the night sky to see that. How can you prove that the Earth turns when you can't feel it? You can see the Sun move round the sky by day and the stars wheel across the heavens at night, but you can't feel it. No wonder it took thousands of years too prove it was us moving and not the sky.
Jupiter looks brighter than Mars, but it's three times further away. The Sun and the Moon are the same size in the sky. By a fluke of nature the Sun happens to be 400 times the size of the Moon but it is 400 times further away, therefore a close fit in size in our sky. A feature which some of us appreciated last August.
William Herschel spent years looking at the night sky through the largest telescopes of the day and he eventually decided that the shape of the universe was a brick like shape with the Sun near the centre. Where there were gaps in the general spread of stars across the Milky Way he assumed that there were fewer stars in that part of the heavens. He never thought that less stars in an area meant a obstruction of the star light by dust or gas. This fact took another couple of centuries until telescopes and photography in the early years of this century proved the existence of clouds of dust and gas blocking the light of distant stars.
Stars that look close together, such as in many double stars, can in fact be many light years apart. The sky contains many pairs of such line-of-sight doubles, stars which are completely unattached to each other. Other stars which do form a binary can vary wildly in size and brightness with many double stars presenting a challenge to observers to split them in scopes.
Constellations with neat arrangements of evenly bright stars are not necessary close to each other or at the same distance from us. A good example is the seven main bright stars in the constellation of Orion the Hunter which are actually spread over 2,000 light years in depth! Lets start with the red star Betelgeuse at the top left, it is 310 ly from us, Bellatrix (top right) 360 ly, Rigel, a large blue B8 giant at bottom right lies 925 ly away and Saiph (bottom left) only 70 ly. The three belt stars which look nearly the same brightness to us are (L to R) Alnitak, 1100 ly, Alnilam 1220 ly and Mintaka a bright O9 star, is a massive 2300 ly away! This star has a luminosity of 53,000 times our sun. You can see from just this one constellation that the shape and brightness of its stars depends very much on where we are looking at it. For instance if we could look down on Orion from above at 90° to how we see it now, we would not recognise it at all! None of the stars would be in a shape we would recognise.
Another example is the Plough, Ursa Major, the seven bright stars of this asterism look similar in brightness but the two stars at each end are not part of the group and lie much further out. Dubhe the pointer star nearest Polaris is at 124 ly, Merak, the bottom pointer is 79 ly, next Phad at 84 ly, Megrez 81 ly, Alioth 81 ly, Mizar 78 ly, Alcor 81 ly and the tail star Alkaid at 101 ly. These distances are the latest from the Hipparcos mission, and you can see that the middle five stars of the Plough are part of the same group and all are moving through space together in the same direction.
Because It Isn't There
The Ascent of the Anti-Matterhorn
by Mike Frost
As always, I try and stick with plausible (ish) physics. The story of Edward Whymper is all true, however...
Perhaps you'll recall, that the last time I met my friend Clive, of the Interplanetary Dangerous Sports Club, he had spun me a fantastic and alcohol-strewn yarn about his exploits in deep space, before promising to spin me another, equally fantastic yarn about his ascent of the anti-Matterhorn, in the nearest pub.
Well who was I to refuse?
We sat in the lounge and tidied up several pints whilst Clive regained his energy and story-telling abilities. By the time we had reached the matter in hand, we were both in a combative mood.
"So WHY didn't I climb the anti—Matterhorn, then?!" Clive demanded of me.
"BECAUSE IT ISN'T THERE!" I snapped back. If Clive was going to foist another of his stories on me he was going to have to be very convincing. "Anti—matter just doesn't exist in any sizeable quantities in our universe. Just a few anti-atoms in the laboratory - and that's it".
Clive looked piqued. "But WHY isn't there any anti-matter in our universe?"
"Because it annihilates itself as soon as it comes into contact with matter. Matter plus anti-matter — BOOM! Complete annihilation in a burst of gamma rays". I thought I'd got him but I was disconcerted to see Clive smile.
"So by your own logic, there shouldn't be any matter either, should there?"
I could see it was going to be a long evening. "First of all there was the big bang, when all there was was light. No matter, no anti-matter, just photons. Then things began cooling down, and matter and anti-matter began to condense out. The matter and the anti-matter began wiping each other out — but there was an imbalance, a slight bias toward matter, meaning that when all the anti-matter had been annihilated, there was still some matter left."
"Maybe", said Clive, "but WHY was there an imbalance of matter? Why not an imbalance of anti-matter?"
"Well", I said, "we think it was simply a random thing, a quantum fluctuation. It could have gone either way." Clive looked doubtful. "Think of it like this. When you sit down for a college dinner, you've never sure if you should eat from the side plate on your right or the one on your left. It could be either. But as soon as one person makes the decision, it forces the issue for everyone else. Either everyone eats from the righthand plate or everyone else eats from the left".
"So", Clive interrupted, "if you pick the left hand plates you get pasta; if you pick the right hand plates you get antipasta".
I wasn't sure he'd completely grasped it. "Well that'll do for starters..."
"But hang on", said Clive, doubtfully, "one table might end up eating from right hand plates, the next table along from left hand plates. So why don't we get some galaxies made of matter and some galaxies of anti-matter?"
"Well, galaxies are quite close to each other, on a galactic scale, and they interact with each other a lot. If there were any anti-galaxies around, we'd know, because there would be the most almighty explosion when an anti-galaxy collided with an ordinary galaxy. We don't see any such explosions — so we can be pretty certain that anti-galaxies don't exist."
"But what about the galactic voids?", Clive insisted, "There are huge great voids in space with no galaxies at all".
"And no anti-galaxies either. In fact, nothing at all. That's why we call them voids..."
"That's where you're wrong, Frosty," Clive said smugly, "because I went to one of them..."
Well, I had to give him credit. He'd come up with something at least faintly plausible. "All right, I said. Tell me about your visit to the anti-Matterhorn... But let me get you another pint first..."
Clive settled back contentedly. "I was in a two-man exploratory craft sailing deep into the Bootes void, hundreds of millions of light years from the nearest light source. I'll tell you, it was eerie..."
I noted he pronounced Bootes correctly (Boh—oh—tees, not booties), which meant he'd at least done some background research for his story. I was pretty sure he wanted to be asked what he was doing there, but that would be playing along with Clive's game. So I asked him who the other chap was.
"Eddie.. Eddie Shackleton-Fiennes... you wouldn't know him. Hell of a chap. Such a waste....". He tutted sadly. "... as I was saying, for weeks on end the sky was a most eerie sight. No stars at all! Just the faint glow of distant galaxies, diffused and indistinct. It was very off-putting. Then one day Eddie spotted something on the long-range scanner.
'Look, old boy, a star! With a planet in orbit round it. Fancy coming across these chaps in the middle of nowhere, eh...'
Eddie was all for heading straight for the planet for a look round, but I was a little more cautious. Thank goodness I was! When we did a detailed analysis of the stellar composition, we discovered, to our amazement, that both star and planet were made of anti-matter! Just think, some distant remnant of the big bang, which had somehow avoided being obliterated by all the matter in the rest of the universe. I was astounded. Eddie was gung-ho..
'Come on then, Clive old bean. Let's go and stake our claim for Britain...'
You'll realise [said Clive] that Eddie was very patriotic, and took the diminishment of the British empire personally. In a previous century Eddie would have been governing India or opening up the heart of Africa. Now he was reduced to looking for anti—planets in the bits of the universe that the Americans had forgotten to explore. I had to tell him one or two basic bits of physics. British or not, as soon as he set foot on the surface of an anti-planet, he would explode as the antimatter beneath his boots annihilated itself with the matter of his body. Eddie was impressed but not down-hearted.
'So can't we isolate ourselves from this anti-matter stuff. You're a postgrad, Clive, old chum, you can rig something up, surely?'
Well I tried my best. We went into orbit round the star, and managed to locate a small asteroid belt. We succeeded in blowing up one of the more anti-iron-rich asteroids by the simple expedient of firing the waste disposal unit at it. I sat down with the main drive plasma field diagrams and figured out a way of keeping matter and anti-matter apart by means of a magnetic containment vessel."
I kept my mouth shut. This was manifestly impossible for Clive, who at times had difficulty mastering an alcohol containment vessel. But if we got into an argument about magneto-hydrodynamic engineering he'd never finish before closing time. I think Clive appreciated my co—operation.
"You see Frosty, it was perfectly safe to deal with anti-matter, so long as we only touched it with electromagnetic entities. Barge poles were out, but laser light and magnetic fields were fine. With the help of the ship's magnetic field I was able to magnetise the iron core of the anti-asteroid, and once magnetised, the plasma containment field could hold it in place, whilst I shaped the anti-matter with the ship's lasers. Easy peasy!
Anyway, Frosty, to cut a long story short, we were able to build a structure consisting of a sheet of magnetised matter and a sheet of magnetised anti-matter of the opposite polarity, held permanently apart by their magnetic repulsion. It was my idea to build some sort of platform which we could land on the anti-planet's surface. Eddie had other ideas...
''Boots, Clive, make some boots! We'll be going for a short stroll when we land...'
So we fashioned the sheets into boots — a bit like waders — into which we could climb in our space suits. Of course we had to take extra-ordinary care not to touch the outside of the boots in the process, but once we had got into the boots, we would be able to clamber around, in relative safety, with our bodies isolated from the deadly anti-matter beneath our feet.
Landing our spaceship on the anti-planet surface was out of the question, of course, but as you know I had some experience in alternative means of arriving. We went into geo-stationary orbit around the anti-planet and lowered a cable — with our waders attached to the end — down to the planet's surface. Then we abseiled down the cable, and slipped into the boots at the surface.
It was quite a moment when we set foot — or at least, magnetically contained foot — on the surface of the anti-planet. There was almost no atmosphere — which was good for us, because it meant that no atmospheric anti-particles could attack our spacesuits. The planet was very mountainous, and just to be on the safe side we'd landed on the highest flat bit we could find. Eddie and I stood at the bottom of the cable and gazed gobsmacked at the jagged peaks surrounding us. After a long silence, Eddie spoke first, gesticulating at the sharpest, highest peak in front of us.
'I want .. to climb .. up THAT one...'
'Are you crazy?!' I cried, 'we're standing on ten gigatonnes of anti-matter and you want to go mountaineering?! Let's take some pictures and get out of here!' But I could sense Eddie had bigger plans.
'No Clive, old bean, this is our manifest destiny. In front of us lies the anti-Matterhorn, and we are going to climb it. For Britain....'
I hadn't suspected Eddie of mountain climbing tendencies — the nearest he'd previously got to mountaineering was driving a Sherpa van. But now he came to mention it, the peak in front of us did look a bit like the Matterhorn. But what did Britain have to do with it?
'Ever heard of Edward Whymper?' Eddie asked impatiently. 'The man who led the first successful ascent of the Matterhorn, in 1865. Not Swiss, not Italian, but British... For heaven's sake man, where's your sense of national pride. Don't you want to follow in the footsteps of our countrymen?... The great expeditions of Captain Cooke, Captain Scott, Frobisher, Drake...'
'They all died, didn't they?'
'Maybe. But not Whymper. His party reached the summit without losing a man. What do you say, Clive, old boy. Shall we give it a go?' 'Well... I don't think it's a wise idea, Eddie...'
Eddie fixed me with a stern stare. 'Clive! Are you a wimp or a Whymper?'
What could I say? No one EVER called me a wimp. '... well, if you say so...'
'Good lad'. He turned round and strode off. 'Of course', Eddie remarked quietly, 'half Whymper's party died on the way down...'"
I waited for Clive to return from the toilet. "So what happened. Did you make it to the summit of the anti-Matterhorn?"
"Well," Clive said, shiftily, "you have to understand it was a real effort. Our spacesuits weren't designed for mountaineering, and the magneto-waders made movement even more difficult. We both carried some things we might need — shovels for digging samples, that kind of thing — and clipped to the outside of the waders were containers to place the implements in, so that they could touch the ground safely. But we had no walking sticks or anything like that. There was no snow, of course, no atmosphere!, but the anti-soil beneath our feet had a clingy consistency, which made walking difficult. And worst of all was the constant risk that the containment field might be breached. One or two atoms of anti-matter weren't a problem. But if just a few milligrams of anti-matter, just a tiny speck of dust, got inside one of our waders, it would cause an explosion which would rip the boot open and lead to our mutual annihilation in an immense explosion. I'm not the most cautious of people, as you know, Frosty, but I thought we were taking a hell of a risk. But still Eddie plodded upwards."
He hadn't answered my question. "So did you make it to the summit?".
He shook his head, sadly. "I got attitude sickness."
"You mean altitude sickness"
"No, attitude sickness — I got sick of climbing. We'd been ascending for hours and making next to no progress. I started complaining to Eddie...
'Eddie, you said we were just going for a short walk. We've been gone for ages.'
'Great Scott, Clive, have you no backbone? We're nearly there!'
'Eddie, we've miles to go, and I'm knackered. I'm going down. See you back in the spaceship..'
Eddie turned and looked at me with piteous contempt. 'YOU may have no spunk, Clive, old bean, but I intend seeing this through... For Britain, old chap, for Britain...'. Then he turned round and carried on relentlessly upwards. I made my way back down to our landing site, clipped myself back on to the cable, pulled myself out of my waders, and made the long, long winch ride back up to the safety of the spaceship. Back in the ship I poured myself a beer and called up Eddie on the communications system. He had a TV camera inside his suit so I could track his progress — and to my surprise I could see the summit of the anti-Matterhorn in sight.
'Clive, old bean, good to hear from you. I'm nearly there...' I could tell he was very tired from his exertions, but at the same time exhilarated by his approaching success. He said nothing for the last few minutes of the climb, but I watched, enthralled, as he made the final steps to the summit.
For just a few moments he gazed down at the vast ranges of anti-mountains, extending away beneath him, as far as eye could see, in every direction. Then he reached inside his pouch, and pulled out a plastic Union Jack, attached to a pole. He held the pole exhaustedly but triumphantly above his head.
"For Queen and Country!!" he cried, proudly, "I claim this planet, and all her domains — for Britain!!!". Then he plunged the pole down towards the soil of the summit.
'EDDIE!' I cried, 'PUT THE FLAGPOLE IN A CONTAINER!!'...."
Clive sipped his pint sadly. "The blast nearly blew me out of orbit..."
Despite myself, I was moved. "How very British. Such glorious failure. Disaster snatched from the jaws of triumph..."
Clive nodded, "but you know what?". I shook my head.
"He ended not with a Whymper but with a bang..."
You may have seen a version of this article in the April 1999 edition
of Astronomy Now. Whilst I was (as always) delighted that Astronomy Now magazine wanted
to publish one of my pieces, I was disappointed that Pam Spence, the editor,
cut out about two-thirds of my original article (not to mention about thirty
per cent of the title!). Whilst the remainder of the article is not
particularly astronomical (which is probably why it was cut out), I think
it is a fascinating story, strikingly relevant to today's world, and I
very much hope you will enjoy reading it.
So may I present the original, uncut, version of
The Accidental Death of an
by Mike Frost
On Thursday, February 15th 1894, Martial Bourdin, a young Frenchman
of diminutive stature, ate his lunch at the International restaurant in
Fitzroy Square, London, then made his way to Westminster, where he caught
the 3.10pm horse-drawn tram to the East Greenwich terminus. His was
the only through ticket, and the conductor later remembered that the Frenchman
seemed nervous and concerned about a package on the seat beside him.
In Bourdin's pockets, it later transpired, were thirteen pounds sterling
in gold (a substantial sum in 1894), a membership card for the Autonomie
club, a ticket for a ball ”in aid of funds for revolutionary purposes",
and pieces of paper on which were written, in Latin, chemical formulae
The tram arrived at its destination at 4.19pm. Bourdin asked the depot
checker the way to Greenwich park. It had been a rainy day and the
light was failing as the Frenchman made his way into the park from King
William Street, shortly before the closing time of 5pm. A gardener
called Burchill noticed him and remarked to a companion about a little
man in a brown overcoat and felt hat, hugging a parcel the size of a brick.
Bourdin made his way across the park on the main avenue, and began to climb
the hill towards the observatory at the crest.
In 1998, of course, the Greenwich Observatory forms part of the National
Maritime Museum, and is no longer a professional observatory, although
a telescope is still available for use by the people of London. But
in 1894, the buildings were still home to the Royal Greenwich Observatory,
an institution of international renown, established on the Greenwich site
for 219 years. Since 1884, Greenwich had been the undisputed site
of the international meridian, the arbitrary line of longitude marking
the transition from east to west hemispheres. The chief purpose of
the observatory was the production of almanacs and astronomical predictions;
to this end, careful measurements of stellar transit times were made in
the observatory and laboriously reduced, by human "computers", into tabular
form. On February 15th 1894 at nine minutes to five, most of the
staff had finished for the day, as Martial Bourdin approached the observatory,
carrying a bomb.
As we shall see, we can only guess at the reasons behind his journey
to the Greenwich Observatory. What is better known is the curious
way in which his story came to be told, first in print by one of the great
novelists of the twentieth century, and latterly on screen by an impressive
array of Hollywood's finest actors. And, recently, his story has
had bizarre echoes in the actions of latter-day revolutionaries.
Martial Bourdin was born in 1868, and moved to London in 1888 to help
his brother Henri, who was a tailor in Soho. Bourdin had already
developed anarchist tendencies in France; in London, he joined the Autonomie
Club, based in Windmill St. off Tottenham Court Road, where English and
foreign anarchists dined and swapped ideas and plots. He was sent
to America in 1891 or 92 on behalf of the anarchist movement, and lived
in New York and Chicago. On his return to London he became a zealous and
trusted member of the movement. There are even reports of him giving
a lecture on explosives to the Autonomie club. Other authors, however,
paint a picture of Bourdin as a trusting simpleton. It is difficult to
gain a clear picture of the man, particularly as his fellow anarchists
had their own reasons to obscure the truth.
On February 12th, 1894, just 3 days before Bourdin's trip to Greenwich,
a bomb was thrown in the cafe Terminus in Paris, killing one man and injuring
others. The French police believed that the materials for the bomb
came from London, but were slow to notify their British counterparts of
their suspicions. The Autonomie club almost certainly knew they were
due for a raid. Perhaps the London anarchists decided to launch a
hastily prepared outrage of their own, or maybe Bourdin was fleeing London
for France, and simply chose Greenwich park as somewhere to dump incriminating
explosives (although this seems unlikely given the number of parks Bourdin
could have chosen closer to central London, and his reported determination
to make an unfamiliar journey). Finally, there is the possibility
that Bourdin was simply duped into carrying the bomb into the park by some
unknown agitator whom he trusted.
Nowadays the path to the observatory from the river slopes from west
to east in front of the observatory buildings. In 1894, no such path
existed, and instead much steeper steps, not in good repair, zig-zagged
up the hill to the west of the observatory. Perhaps Martial Bourdin
tripped on a tree root, perhaps he fumbled as he tried to prime the bomb
with a phial of acid. Whatever the reason, Bourdin had only reached
the second bend from the top when his bomb went off prematurely.
The aftermath of the bomb
W. G. Thackery and H. P. Hollis, two assistants, were talking in the
lower computing room, when, to quote Thackery, "we were suddenly startled
by a loud explosion, the detonation of which was sharp and clear, apparently
followed by a noise something like a shell going through the air.
I immediately remarked to Mr Hollis That is dynamite! Spot the time".
The explosion was heard throughout the park, and several people came
running. First on the scene were two schoolboys, George Frost and
Thomas Winter. They found Bourdin, covered in blood, glass and black
powder, kneeling on the pathway, unable to walk away. He called to
the boys "Come here! Come here! Call me a cab", but the boys were too scared
to come close.
Shortly afterwards park-keeper Sullivan arrived and a park constable
came running up the path from the avenue. Again, Bourdin cried out "Take me home, take me home", in a foreign accent. The constable noticed
that he had wrapped a handkerchief around his left hand. Fearing
that the bomber carried a revolver, he removed the handkerchief, and was
horrified to find that the Frenchman's left hand had been blown off clean
above the wrist. Dr Willes, who lived nearby in Crooms Hill, was
summoned. He opened Bourdin's waistcoat and found that the poor man's
stomach had been blown open. Bourdin was taken by stretcher to the
Seaman's hospital at 5.15pm, where he lingered, to the surprise of the
doctor, for 25 minutes, and then died.
The attempted bombing caused a sensation, occupying the press for several
days. It was reported that "excitement at Greenwich is very great
and many people have made their way along the zig-zag path where the accident
occurred". Park keeper Sullivan revelled in the publicity, remarking
that "in all his experience he had never known so many people in the park
on a single day, except on Bank Holidays, even when the band was playing
in the summer months."
Although some eye-witness accounts spoke of damage to the observatory,
in fact there was none a trail of blood and bone fragments stopped
well short of the observatory railings. Extra guards went on duty at the
Woolwich Arsenal, Dockyards and Barracks, and the Autonomie club was raided
and documents seized.
On Monday February 19th, an inquest took place in the Lecture Hall,
Royal Hill. Several Autonomie club members were present, also Martial's
brother Henri, who created a stir by refusing to swear on the bible.
When the inquest jury visited the Greenwich park, they were met by a 'decently-dressed
young fellow' who cried "Vive L'Anarchie!", and denounced the existing order.
When asked to desist, he walked off, making uncomplimentary remarks about
the then Home Secretary Herbert Asquith. Colonel Majendie, government
inspector of explosives, carried out an enquiry into the security of the
observatory and concluded, after some deliberation, that nothing needed
to be done.
The Home Office took steps to ensure that Bourdin's funeral was not
a rallying point for the anarchist cause. The police had to protect
the few mourners who did attend from a crowd who attacked the cortege in
Fitzroy Square. Martial Bourdin's spectacularly unsuccessful career
should have ended with his burial, on Feb 23rd 1894, in St Pancras cemetery,
Finchley. Yet his story was to undergo an unexpected renaissance.
Over the next thirteen years, the Greenwich Park bomb faded from memory.
Then, in 1907, the writer Joseph Conrad (Polish born but living in England)
happened to discuss the story with a friend. Conrad recalled he had
been appalled by the futility of Bourdin's gesture, "a blood stained inanity
of so fatuous a kind that it was impossible to fathom its origin...".
His friend happened to remark that Bourdin had had a sister who committed
suicide after Bourdin's demise. Whether or not this was true is not clear;
there is no mention of a sister in contemporary accounts. But Conrad
was struck by the effects of an essentially political act on family relations.
The grain of an idea formed from his discussion to retell the story
of Bourdin's attack on the Greenwich Observatory from the point of view
of the bomber and his family.
What emerged was a novel called "The Secret Agent", which is considered
one of his finest. The nihilistic attack on the observatory remains
at the heart of the novel, though it is not described explicitly.
However, for dramatic purposes, the characters have been developed somewhat
from the original story.
The Secret Agent
The Secret Agent of Conrad's book is Adolf Verloc, a shopkeeper who
lives with his wife Winnie and her simple-minded brother Stevie, above
a London book-shop selling dodgy material. Verloc is the agent of
a foreign power (never named) and was once the provider of military secrets,
but has now no wish other than to live in domestic comfort. His one
contribution to international politics is to run a society, the grandly
named Future of the Proletariat, who are in fact nothing more than a talking
shop. Verloc's nationality is never explicitly defined, but it is
interesting that Conrad makes one member of the secret society, Alexander
Ossipan, a French anarchist.
Verloc's comfortable existence is disrupted when he is called to his
employer's embassy to be torn down a strip by the first secretary, Mr Vladimir.
The diplomat is displeased by Verloc's lack of action and demands that
he carry out an act of terrorism. Perhaps, in Vladimir's choice of
target, we gain an idea of Conrad's view of the Greenwich incident.
The intended site of the outrage must be a building, not an individual.
Royalty and religion are ruled out, as is Verloc's suggestion of a bombing
campaign against the embassies. Instead, Verloc' controller has a
more precise target:-
"..The sacrosanct fetish of today is science... what do you think of
having a go at astronomy?"
Why astronomy? "I defy the ingenuity of journalists to persuade
their public that any given member of the public can have a personal grievance
against astronomy ... the whole civilised world has heard of Greenwich.
The very boot-blacks in the basement of Charing Cross Station know something
Verloc, shaken out his complacency, is forced to plan the bombing.
He consults the mysterious Professor, a one-time chemistry technician,
who, slighted by his superiors, has withdrawn in contempt from society,
and now harbours a hatred of all institutions, and scientific institutions
in particular. The Professor (drawing on another facet of Bourdin'
character) provides Verloc with the materials for a bomb. But instead
of planting the bomb himself, Verloc imposes on his dim-witted brother-in-law
Stevie to do the deed. It is Stevie' clumsiness which leads to the
bomb exploding prematurely, and Stevie' death which drives the story to
its tragic conclusion.
Conrad's novel was a great critical success; Penguin Popular Classics
bill it as "arguably one of the greatest novels of the twentieth century". Chillingly, it has been reported that Theodore Kaczynski, the UnaBomber,
drew on the novel as inspiration for his bombing campaign, identifying
with the antisocial and anti-scientific character of the Professor.
Kaczynski allegedly used the pseudonyms Conrad and Konrad when on his bombing
The Secret Agent has been filmed twice. Alfred Hitchcock was first in
1936, under the title "Sabotage" (NOT "The Secret Agent", which was a later
Hitchcock film). "Sabotage" was a free adaptation of the book, with
no mention of the Greenwich Observatory the bomb goes off accidentally
in a bus on the Strand (which co-incidentally was what happened to an IRA
bomb over sixty years later).
More interesting, perhaps, is a more recent and star-studded version
directed in 1996 by Christopher Hampton, famous as the director of "Dangerous
Liaisons" and its stage predecessor "Les Liasions Dangereuse" (he also wrote
the screenplay for "Total Eclipse" which, despite its name, has emphatically
no astronomical connections). In the U.K. this was released (by Fox
Entertainments) as "The Secret Agent", and in the U.S. under the more pedantic "Joseph Conrad's The Secret Agent".
One has to wonder what Bourdin would have thought of the string of household
names employed to tell his story would he have been flattered by
the attention or contemptuous of the establishment film industry?
Verloc is played by Bob Hoskins; Ossipan, the french anarchist, by Gerard
Depardieu; the Professor by Robin Williams (under the pseudonym George
Spelvin). The support cast list is no less impressive Mrs Verloc
is played by Patricia Arquette, Stevie by Christian Bale and the first
secretary Mr Vladimir by Eddie Izzard. The film features a score
by minimalist composer Philip Glass and was filmed on location in London
and Loughborough, including scenes in Greenwich Parkand at the Greenwich
With such an impressive pedigree it is a great shame that the film
was a critical and box-office failure. Perhaps the relentlessly downbeat
nature of the story counted against it; some reviewers criticised the non-linear
flow of the story, which followed the book' structure. However, it
would be a pity to miss the dramatisation of one of astronomy' more unlikely
brushes with politics.
Watch out for it on TV!
The author would like to thank Maria Blyzinsky of the National Maritime
Museum, Adam Perkins of the Cambridge University Library and Philip Taylor
of the Royal Greenwich Observatory for their assistance in researching
Royal Greenwich Observatory archive RGO 7/58, Cambridge University
Fireworks at the Royal Observatory by Philip Laurie (The
Castle Review, Journal of the RGO Sports and Social Club, Vol 5, No 1,
Propaganda by Deed The Greenwich Observatory Bomb of 1894
by Philip Taylor (Open Space No 9, P4, Nov 1996, PPARC)
The International Anarchist Movement in Late Victorian London
by Hermia Oliver (St Martin' Press, 1983)
Unabomber 'Based his Life on Novel' by Serge F. Kovaleski (Washington
Post, reprinted in Guardian Weekly July 21st 1996)
The Secret Agent by Joseph Conrad (first published 1907, Penguin
Popular Classics 1994)
by Paritosh Maulik
Dust not only gathers on our book selves and on our windowsills, but
it is also out there in space! It is present everywhere, so
rejoice, do not feel guilty of not dusting; read about it.
There is a lot of dust about! In the inner solar system, the Zodiacal
light is a faint brightening or glow in the sky just after sun set and
just before dawn; the French astronomer Cassini wrote about some aspects
of this phenomenon. Now we know that it is due to the scattering
of the suns rays by the dust in the same orbital plane as the planets.
Scattering is the change in the direction of motion of a photon, almost
without energy loss.
Sir William Herschel observed some gaps in the distribution of stars
on the sky and thought that these where just holes in the halo of stars
around us. Then around the 1930's and 40's, when observation techniques
and the theory of the universe became more refined, it was noticed that
the distance between the stars in a cluster depends on the method of measurement.
It can be calculated, A, either from the luminosity of the stars or B,
the linear dimension of the star. It soon became apparent that the
distance estimated from method A was greater than if estimated from method
B and therefore, if something present in the interstellar medium absorb
the stars light (obscuration), giving the impression the star being fainter
and further away.
Further observations based on obscuration at different optical wavelengths,
suggested the presence of solid particles of size similar to the optical
wavelength i.e. about 0.1 - 1 µm (1 µm = 1x10-6 m). Advances
in infrared spectroscopy revealed further properties; these small solid
particles remained no more the bet noir of the astronomers for forming
the smoke screen; instead these were hailed as:
I) the building blocks' of
II) formation of molecules in the
III) material rejected by stars in its
IV) the formation of planetary system
around the stars.
Now we know that the gaps in the star clouds noticed by Herschel is
caused by obscuration by dust. Some of the current information of
this cosmic dust is discussed here.
Detection of Dust Particles, Spectroscopy
When electromagnetic radiation strikes a dust particle, the atoms of
the particle may absorb the energy, go to an unstable exited state and
then eventually fall back to their normal state by releasing the excess
energy in the form of emission spectrum from the dust particle. However
sometimes the dust/gas cloud may absorb the incoming radiation, but the
energy rise of the dust/gas is not enough for it to emit any spectrum.
The incoming spectrum now misses some wavelengths which appears as a few
missing bands — the absorption spectrum. These spectroscopic
studies are useful in understanding the chemistry of the dust/gas cloud.
But an observer, whether in space or on terra firma, measures the combined
loss of radiation — caused by scattering and absorption; this is called
Particles also scatter the incoming radiation (or wavelength).
For particles to scatter radiation, the wavelength and the particle size
has to be comparable. By studying the scattering process one can
get information on the size of the particle.
In the early days, terrestrial based observation of the spectrum of
astronomical objects identified the presence of cosmic dust. A wide
range of radiation, from X-ray to infrared to millimetre range radio waves,
were tried. The major obstacle to the terrestrial based observation
is that the incoming radiation is either reflected back or absorbed by
the everyday compounds in the atmosphere like water vapour, oxygen etc.
Now, orbiting satellites above the earth's atmosphere can give a more elaborate
picture. At one time x-ray films sent on rocket missions, lasting
for a few minutes, was the state of the art. Physical scientists have worked
out the relationship between the wavelength, emissivity, absorption, particle
size and the temperature; thus we can choose a suitable wavelength, to
study a given set of properties of dust.
Below is a short list of radiation ranges used for the study of the
dust and the information these can provide.
Millimetre wavelength range 0.35 to about a few mm
Emissivity and hence the temperature of the dust grain, and information
about the molecules both circumstellar (around stars) and interstellar
(general background) region. If a star is far away, radiation takes
long time to reach us, indicating an earlier epoch. If the dust from
such a region is heated by the radiation from the star it emits infrared,
but by the time these have reached the earth, the wavelength has been red
shifted to a longer wavelength than the infrared; into the sub-millimetre
wave length. Earlier optical studies suggested that the peak rate
of star formation took place when the universe was about three-quarter
of its present age. However, recent sub-millimetre wave length observations
at the James Clerk Maxwell telescope in Hawaii indicate that the peak rate
was much earlier (Modern Astronomer, Vol. 2, September, 1998, p 284).
Infrared range 1x10-3 to about 1x10-6m
Temperature (from emissivity data), absorption or emission spectrum
in the infrared helps to identify composition and the scattering data give
an indication of the grain size of the circumstellar dust.
Optical range 7x10-7 to about 4x10-7m
It was the observations in the optical range, which first alerted the
astronomers to the possible presence of dust obscuring the halo of stars.
For example, if one area of the sky is viewed with different filters, the
images appear somewhat different. This is caused by parts of the
spectrum from the dust/gas cloud being absorbed by the filter. If
the light from a cluster of stars passes through the interstellar dust,
it may illuminate it. Diffused glow of the sky is due to the scattering
of starlight by the dust. Individual stars are sometimes surrounded
by a dust cloud; this dust cloud was either generated by the star or is
leftover matter from the star formation. In general, the optical
range perhaps does not provide much quantitative information like the size
of the dust particles, none the less, it, often gives information about
the chemical nature of the dust.
Ultraviolet range 1x10-7 to about 5x10-10m
Hydrogen gas is very abundant in the universe and it absorbs radiation
in the ultraviolet range, so the information does not reach the ground-based
observer. However detectors on board spacecrafts have produced useful
information about both circumstellar and interstellar dust.
X-ray range 5x10-10 to about 5x10-13m
Spectrum in the range of 0.1 - 100A° (1A° = 10-8mm) is examined.
Radiation in this range is not absorbed by hydrogen gas. For a dust
grain to emit x-ray radiation of a wave length of about 30A°, its temperature
has to be around 106k; at this temperature the dust should evaporate. However
if the dust cloud is near a x-ray source, it can scatter x-rays, and hence
can point to the source. X-ray observations studies have been useful
in the study of interstellar dust. The bombardment of ions from a
supernova explosion can increase the temperature of the dust to emit x-rays.
Beyond the x-ray is the gamma ray, 5x10-13 to about 5x10-16m
wavelength range. Gamma ray sources are well known to astronomers,
but these have not been used to study the cosmic dust.
To sum up:
i) Dust grains in the range of about
1- 100µm are responsible for the zodiacal light and to detect dust
particles 100µm and below, spectroscopy is the tool.
ii) Particles above about 100µm do not
take part in the commonly observed phenomenon.
iii) Particles above 100 to 500µm leaves an
ion trail when entering earth's atmosphere.
iv) Heating and ionisation of gases on entering
earth's atmosphere is caused by particles in the range of about 500µm
and greater; shooting stars or meteor shower are particles upwards to several
metres ˜ entering the earths atmosphere.
After learning about the spectroscopy and the scattering process, astronomers
attempted a close encounter with the dust by sending space probes to be
stoned or bombarded by dust. These studies were looking for those
grains, which are too small to be detected by the spectroscopic methods.
Although these may be small, but they make their presence felt — Giotto mission to Hally's comet suffered from impact damage; some of the
instruments stopped working, the space probe was tilted so that commucations
were temparaly lost. This was probably caused by larger particles.
All this information has other importance as well. An interplanetary
spacecraft may suffer from an impact velocity of the order of 10 to 20km/s,
whereas for a fly by or sling shot situation, the impact velocity may rise
to about 70km/s. It is expected that in future solar probe mission
may encounter impact velocity of about 310km/s. These probes are
expected to provide information from about 4 times solar radius distance
from the centre of the sun and if the sun is to act as gravitational sling
shot, this information would be useful.
For real proof of the presence of dust grains in space:- the lunar
rock brought back by the Apollo missions showed micron size crater impacts
from small particles. Such a dust detector essentially consists of
about 0.1mm thick foil, hemispherical in shape. The maximum sensitive area
is about 0.1m2. When a dust particle strikes the detector, plasma is produced.
There is an arrangement of electrodes, which separate the charge into constituents
such as electron, positive and negatively charged irons. These are
analysed to obtain information not only on the chemistry of the dust particle,
but on its velocity as well.
These instruments are calibrated in the laboratory before being put
on board the space probe. This is carried out by bombarding the instrument
target with particles of a known mass and charge range one is expected
to meet; the angle of impact is also an important consideration.
Part of the Galileo programme was aimed to study the mass, velocity
and electric charge of particles in the range of 10-16 to 10-6g in the
Jovian atmosphere. The information obtained would provide information
on the nature of the particle and interaction with magnetic and radiation
fields and their role in forming the ring around Jupiter.
Now that we now how to look for dust, let us see what we know about
Origin of Dust
Roughly speaking the comic dust is of two types:
i) Circumstellar: associated with a single
ii) Interstellar: uniformly distributed throughout
The presence of the circumstellar dust is either due to
A: the star is relatively young and the observed
dust is remnant material from the star formation,
B: stellar wind (a radiation induced movement
of fine particles) has attracted the dust around the star.
There is also a third possibility, chemical reaction occurring
in some of the mature stars can lead to the formation of dust around the
Since this type of dust is associated with that of a single star, its
properties are dominated by the star itself. The temperature of the
dust is higher than the interstellar background. The circumstellar
dust shell is of the order of about 1pc (the distance, at which the object
would form 1 second of arc ~ 3.09 x 1016m = 3.25 light years). If
we know the optical spectrum of a given star, we can calculate theoretically
the expected temperature and the expected infrared radiation from the star;
but when we measure the infrared radiation from the star, it sometimes
turns out that the star is emitting excess infrared radiation (this is
called infrared excess). This infrared excess is a combined effect
of infrared radiation from the star and the surrounding dust.
Chemistry of Interstellar dust
There are two types of interstellar dust distributions,
i) general distribution of dust in background
ii) dense cloud.
Dust in the interstellar cloud may settle on a core of ice, thus making
the dust grains appear larger than commonly observed in the general background.
We have discussed that the interstellar dust may appear to radiate more
in the infrared regions.
Absorption and extinction behaviours for various galaxies show a similar
pattern; this tends to suggest that the interstellar dust medium is generally
uniform in nature. The mathematics of extinction in the optical region
(7x10-7 to about 4x10-7m wavelength) gives us the following mass or density
distribution (in kg m-3) that can be expected for interstellar matter.
Small particles <<wavelength>> 1.5x10-27
Particles ~ wavelength>> 1.5x10-27
Particles >> wavelength 1.2x10-23
If we assume that the composition of the sun, in weight as, 73% hydrogen,
25% helium and 2% heavy elements, (mass greater than those of hydrogen
and helium), to be the average composition of the gases in the interstellar
media. Now if we calculate the density ratio of dust to gas, it turns out
that the only way we can account for the heavy elements such as carbon,
nitrogen, oxygen, magnesium, silicon, sulphur and iron, if these are present
as chemical compounds.
In this simplistic model, heavier inert gases such as neon are not
considered. However there is one problematic compound, water ice;
it is not present in the interstellar medium, but it can form, for example,
by nucleation of water ice on a grain e.g. a silicate grain. The
ice can form by direct reaction between hydrogen and oxygen or by accretion
of OH radicals to form ice.
Spectroscopic observations tend to show somewhat lower concentration
of heavier elements (i.e. elements other than hydrogen and helium), than
we can expect from theoretical calculations. It has been suggested
that these elements are tied up in gas, for example carbon monoxide, CO,
or as compounds as in dust grains.
Often the spectral pattern of the interstellar dust is not completely
understood; this makes positive identification of the dust somewhat difficult.
It is now agreed that the major constituents are carbon and silicates,
however these may not be present as separate particles but as one compound
nucleating and growing on another particle.
The MNR model, (theoretical calculations by Mathis, Nordsieck and Rumpel)
suggests the interstellar dust grain size to be of the order of 5
- 250nm (5x10-9 - 25x10-8m), (weighing about 10-18 to 10-13g). In
this calculation it is assumed that this fine size distribution is the
result of inter-particle collision. But such a collision process
is difficult to imagine, since the dust density is very low (number of
grains about 10-6 m-3). It is now proposed that the dust grains are
first formed by the stars, then break down to smaller sizes and eventually
travel to the interstellar medium. Ulysses and Galileo probes have
recently identified bigger dust grains 3x10-13 to 1x10-7g. Calculations
based on the availability of hydrogen and other heavier elements, (the
latter take part in the dust formation), suggest that to form the commonly
available interstellar grains of about 0.1µm size, the time needed
is of the order of 1010 years, which is far higher than the age of the
universe; this also indicates that the origin of the interstellar dust
is perhaps somewhere else.
The interstellar dust can lead to some interesting phenomenon such
i) a near by star can light up the dust grains:
this is called reflecting nebula,
ii) light echo from variable stars: the direct light
from the star and the light scattered by the dust appear at different times.
To the observer the scattered light appears to come from a circle, the
radius of which increases with time and the net effect is like an expanding
ripple in a pond. The magnetic and electric fields and the ionised
state of the particles often cause the particles to be aligned in certain
direction. This can give rise to the light (radiation) beam to appear
We have seen earlier that this is associated with a given star and
like the interstellar dust, the circumstellar dust can also make the star
appear cooler, that is, more to the red end of the spectrum. The
presence of this dust reduces the luminosity of the star and by examining
the spectral properties we can determine the nature of the dust particles.
We have discussed earlier that the circumstellar dust can either be
matter condensing to form the star or remnant matter after the star formation.
From the temperature around stars, around 2000K (1700°C) we can expect
the formation of carbon monoxide, CO. After the formation of CO, we can
have excess of, either carbon, or oxygen atoms and if we have excess oxygen,
compounds of oxygen such as oxides of say aluminium, magnesium, iron, silicon
or silicates, (silicate is a oxygen-silicon compound) form. The spectral
signature from silicates prepared in the laboratory and from the circumstellar
dust show only partial resemblance; it is not always possible to say with
confidence, if these silicates are crystalline; however the silicates in
comets appear to be crystalline.
Let us now consider the stars where we have carbon excess. As
the star ages, the nuclear fusion reactions within the star synthesise
carbon and silicon which move to the outer region of the star and the spectral
lines suggests the possible presence of silicon carbide, SiC, molecules.
Some spectral data has been explained as the presence of about 50A°
size carbon particles, but its confirmation is open to interpretation.
(Silicon carbide is almost as hard as diamond, and a lot of it is produced
in the factories for making grinding tools).
When a collapsing protostar collects material from the accretion disc
and forms a star, the material from the disc flows out at right angle to
the disc. The presence of strong magnetic field may cause the dust
grains to align preferentially and this may lead to polarisation.
As the gas and the dust pass between the observer and the star, the pre-main
sequence stars appear to be dusty, and the polarised nature of the beam
may make the stars to appear as variable stars.
When the molecules form, these are initially in the gaseous state;
these then grow by the process of nucleation and growth. The nucleation
is by heterogeneous nucleation i.e. gaseous molecules forming on another
dust grain and then it slowly grows. The situation is similar to
seed crystal used to grow larger crystal in chemistry laboratorys.
This model can explain some of the features of the dust formation around
the star. As early as the 1930s it was recognised that novae (a white
dwarf star in a binary star system, on the surface of which, a runaway
thermonuclear reaction is occurring and material is being thrown out) are
the sites for the formation of dust and gas. By late 1980s some interesting
observations were made. The optical luminosity of the novae goes
through a dip and with time recovers again. This was explained by
the fact that
i) molecules like CN formed, and
ii) these molecules obscured the novae.
Additionally dust also formed and it caused further obscuration of
the novae. The problem with this explanation was, molecules in the
gaseous state can not block out a wide range of wavelengths; so if we keep
on looking at all wavelengths, then at certain wavelengths, we should we
should not see any obscuration.
Finally the infrared observations suggested the sequence to be:
i) as the dust forms, it covers the surroundings
and the star appears dim;
ii) then the dust absorbs some of the radiation
and re-emits in the infrared range;
iii) on dispersion of the dust cloud the optical
luminosity is restored again.
The infrared luminosity therefore first decreases, rises and then finally
drops. To put things in perspective a few numbers, about 1022kg of
material may be thrown out in such an explosion, once it has reached about
1013m from the star, nucleation and growth process forms dust grains of
size of about 0.5µm and the whole process may take 150 to 190 days.
A similar phenomenon also occurs with supernovae, but the explosion
is on a much higher scale; the molecules flow out with greater speed and
it takes longer, about one year, to form dust grains and form particles.
It may be mentioned here that the light echo as discussed with the interstellar
dust may start its life as circumstellar dust around a supernovae.
There is a special group of stars called R Coronae Borealis, which
are rich in carbon molecules and not rich in hydrogen, the most commonly
occurring element. These show an optical wavelength dependent absorption,
but no such absorption spectrum is observed in the infrared range.
Only the novae explosions give rise to specific isotope ratios in the
dust grains and some of the meteorites also show specific isotope ratios,
therefore it may be possible that the meteorite had its origin in the given
In addition to the inorganic compounds like silicates, oxides or silicon
carbides, spectroscopic evidence suggests the presence of organic molecules.
Positive identification is not always possible, all we can say is the presence
of carbon-hydrogen, C-H, bond. In general, both the interstellar
and circumstellar dust is about 1µm in size; this corresponds to
about 3 - 4x106 atoms. The problem of positive identification
is due to the fact that in the laboratory we deal with a much higher concentration
(hence size) of the molecules and we do not know how the spectrum would
look like if we have only a very small amount, say about — 100 atoms in 10A°.
While trying to correlate the possible grain size and the observed
and predicted temperature from spectroscopy, it emerged that about 10A°
size stellar dust particles can be intermittently heated to a higher temperature
due to being bombarded by gas jets. Since the matter is not
plentiful, particle temperature would rise and then fall again before the
next strike. This process makes these particles hotter than if these
are heated by radiation alone. Now, if this calculated temperature
is the true temperature of the dust particles, compounds like water ice,
silicates would evaporate.
However one group of carbon-hydrogen compounds like (C6H6)n, C24H)12,
can explain quite a few of the features of the infrared spectrum.
Some of these compounds can also occur as cluster and platelets.
These compounds are called Polycyclic Aromatic Hydrocarbon or PAH.
Again like many other observations, some of the features remain unexplained
and part of the reason is that the compounds synthesised in the laboratory
may be similar but may not be the same. These molecules might have
been responsible for starting the carbon based life as we know it.
Another well known candidate is large carbon molecules, C60 and C72 — Bukminsterfullerin. Its origin has been explained by the radioactive
decay process in the stars. The structure looks like a soccer ball.
(Theoretical calculations predict that materials based on this molecule
can bring about a new generation of strong materials.)
Dust nearer to home (astronomical scale)
We now know that the dust from the nucleus of the comet scatters sunlight
and makes it visible. The radiation pressure causes the dust to fly
away from the nucleus of the comet. The heat of the sun causes release
of the volatile matters. Some of the dust particles are charged, i.e. ionised — (remember the ion trial of Hale-Bopp). This way comets may account
for the dust particles in the inner solar system. Fragmentation of
asteroids and meteors and mutual collision is the source of dust nearer
to Earth. If this fragmentation of the larger bodies into smaller
particles is the source of dust, then it is likely that the dust in the
solar system is of recent origin and not of the same age as the solar system.
Some of the solar dust, under the influence of solar radiation pressure,
will leave the solar system and move into interstellar space. As
the solar system moves through interstellar space, some of the interstellar
dust enters the solar dust system, for example Kuiper belt (icy planetisimals
just beyond the orbit of Pluto, confirmed 1991 - 92), and may also take
part in further fragmentation of solar dust. Dust particles under
the influence of electric and magnetic forces can align themselves and
eventually form planetary rings. Some recent observations suggest
that the rings around the Jupiter have been formed by micrometeorite collision
with its innermost moons.
We can see that by getting to know the cosmic dust we can learn about
the evolution of the stars, planetary rings, planets and eventually us.
They knew it all along when they said dust to dust. We can not say
anymore obscured, buried under a cloud of dust. We can
now see through the dust. We have different wavelength ranges to
look into the dust cloud, each complimenting the other. Ground based
observations are just important as those aboard spacecraft. We can
now send probes to interact with the dust and may be able to bring back
samples as well. At last we have learnt to treasure the dust.
Dusty Universe Evans, Aneurin, Praxis Publishing Ltd, 1994
Graphs, Amara personal communication
EOS, number 4 Mann, Ingrid, 27 January 1989
The Cassini/Huygens Mission Clarke, Ivor, MIRA 43, 1997
Thanks are due to Ivor Clarke for suggestions on the manuscript and
daughter Twisha for reading the proof..
The Red, Green or Blue debate
by Clive Rogers
Which colour is best for reading star maps but retaining the night vision?
The e-mails below are taken from a mailing list for amateur telescope makers.
For night vision in humans, I think rhodopsin bleaching may be the
crucial issue that makes the question something other than a simple "most
sensitive colour (green) is best". It is certainly possible that faint
green is better than faint red, and informal experiments I have done suggest
it may be so. But I wonder if anyone on the list is aware of carefully
controlled scientific experiments that answer the question exactly how
we astronomers are asking it.
Well, how exactly ARE we visual astronomers asking that question?
I'd ask it something like "what kind of light lets us read the fine print
on star charts and still affect scotopic (averted) vision as little as
If it were just a matter of different over all sensitivity of the rods
and cones, the matter would be simple. I can read a sensitivity difference
of about 5.2 magnitudes at 500nm, 3,8 at 550, 1.6 at 600 and about 0.2
at 650 and above red would win hands down, but it seems that's the
answer to an altogether different question. The fundamental mechanism
of averted vision IS rhodopsin bleaching and as I understand it,
it is the equilibrium between bleaching and regeneration that governs the
I just did a web search, and got the maximum sensitivity for the 3
cone colours: 420-445nm, 535nm, and 564-570nm respectively (surprise! monochromatic
light in this latter range should look green, perhaps yellowish).
It is common experience that the visual resolution is decreased as
the illumination is lowered at dusk, there comes a time when you
cannot read the news text, but you can still easily read the headlines.
It seems likely that the colour monochromatic or mixture is
of greatest importance here, and I suspect this is why many prefer green
light to red. But how about yellow? And is there a difference between
fairly monochromatic yellow from a LED and the minus-blue mixture from
a white light with a yellow filter? It may have to do with the eye's
optical correction for colours, but this would be simply bypassed by reading
A paraphrase of the question: With light of a certain colour formula,
of just enough intensity to barely see by averted vision, how much do you
need to increase the intensity by to be able to read fine print?
The less the better. Does the type of object observed affect requirements
of the chart reading light? Averted vision should be black/white,
but is there still a colour component? Does using a red light bleach
your red sensitivity, bad for observing red objects (i.e., Mars, Lagoon
Nebula, North American Nebula)? Does using green affect observing
This suggests an experiment: take some LEDs of different colours and
connect one at a time via a current meter (you may need a non-dark-adapted
friend to read it!) to a potentiometer to get a variable current
the light intensity should be directly proportional to the current.
Rig it up in a dark room, above a page of fine print (why not of variable
sizes?) OR a white dot on a black background (order of magnitude: 10mm
if seen from some 600mm). Get well dark adapted (I believe you would
need another 5 or 10 minutes after being adapted to a dark country sky,
or between successive tries), then slowly increase the current till you
can barely see the white spot by averted vision, and note the current.
Then put the fine print under the light (see it from a fixed distance)
and increase the light till you can read the print (by direct vision).
This should give you an indication of the margin for that light.
You may then try a mixture of colours and see if this can improve things.
I'm going to try this and see what happens it may take a bit
of cut-and-try to get the currents of the right orders. Anyone else?
This subject will be continued in the next issue of MIRA, if you have
any comments about this, please write in to the Editor.