While Galileo did not injure his eyes by observing sunspots, some other early observers did. Most of these injuries were minor, such as long-lasting after-images. For example, Thomas Harriot, who discovered sunspots independently at about the same time as Galileo, but failed to publish his observations, observed the Sun about noon in February, 1612, and then found “my sight was after dim for an houre.” [See “Thomas Harriot, Renaissance Scientist,” edited by John W. Shirley (Clarendon Press, Oxford, 1974), pp. 129ff, for a discussion of Harriot's sunspot observations. Harriot's observation mentioned here is on p. 140.]
Rigaud quotes Harriot's remark on p. 34, and Greaves on p. 33. He cites Greaves's Misc. Works, Vol. II, p. 508 as the original source, as follows:
Greaves says, that in measuring the diameter of the sun he hurt his sight, “insomuch that for some days after, to that eye, with which I observed, there appeared, as it were, a company of crows flying together in the air at a good distance. At the first I did verily believe I saw a company of crows flying in the air.”
Newton gave a detailed account of his experience some 27 years later in a letter to John Locke [note his use of the y-form of the letter thorn; thus “ye” is to be pronounced “the”]:
The observation you mention … I once made upon my self with ye hazzard of my eyes. The manner was this. I looked a very little while upon ye sun in a looking-glass wth my right eye & then turned my eyes into a dark corner of my chamber & winked to observe the impression made & the circles of colours wch encompassed it & how they decayed by degrees & at last vanished. This I repeated a second & a third time. At the third time when the phantasm of light & colours about it were almost vanished, intending my phansy upon them to see their last appearance I found to my amazemt that they began to return & by little & little to become as lively & vivid as when I had newly looked upon ye sun. But when I ceased to intende my phansy upon them they vanished again. After this I found that as often as I went into ye dark & intended my mind upon them as when a man looks earnestly to see any thing wch is difficult to be seen, I could make ye phantasm return wthout looking any more upon the sun. And the oftener I made it return, the more easily I could make it return again. And at length by repeating this wthout looking any more upon the sun I made such an impression on my eye that if I looked upon ye clouds or a book or any bright object I saw upon it a round bright spot of light like ye sun. And, which is still stranger, though I looked upon ye sun wth my right eye only & not with my left, yet my phansy began to make ye impression upon my left eye as well as upon my right. For if I shut my right eye & looked upon a book or the clouds with my left eye I could see ye spectrum of the sun almost as plain as with my right eye, if I did but intend my phansy a little while upon it. For at first if I shut my right eye & looked wth my left, ye spectrum of ye Sun did not appear till I intended my phansy upon it; but by repeating this, appeared every time more easily. And now in a few hours time I had brought my eys to such a pass that I could look upon no bright object with either eye but I saw ye sun before me, so that I durst neither write nor read but to recover ye use of my eyes shut myself up in my chamber made dark for three days together & used all means to divert my imagination from ye Sun. For if I thought upon him I presently saw his picture though I was in ye dark. But by keeping in ye dark & imploying my mind about other things I began in three or four days to have some use of my eyes again & by forbearing a few days longer to look upon bright objects recovered them pretty well, thô not so well but that for some months after the spectrum of the sun began to return as often as I began to meditate upon ye phænomenon, even tho I lay in bed at midnight wth my curtains drawn. But now I have been very well for many years, tho I am apt to think that if I durst venture my eyes I could still make ye phantasm return by the power of my fansy.
[The quotation above is taken from “The Correspondence of Isaac Newton,” vol. III (Cambridge Univ. Press, 1961) pp. 153-154. The matter is discussed more briefly by Richard Westfall in his book on Newton, “Never at Rest” (Cambridge Univ. Press, 1980) pp. 93-94. Westfall comments that “Newton left the sun alone after that”.]
Newton's description of his symptoms closely agrees with a typical mild solar scotoma: the damage was not evident immediately, but only after several hours; the symptoms were most visible for a few days, and gradually subsided over a long time (several months), eventually disappearing entirely, or nearly so. The photophobia (in which the victim avoids the light) is also a common symptom.
Furthermore, François Arago, in his short biography of Cassini [see Arago's OEuvres Complètes, Vol. 3 (Gide et J. Baudry, Paris, 1854)] says that Cassini “completely lost his sight” [perdit totalement le vue] in 1711. But, as the Dutch professor of ophthalmology M. E. Mulder has pointed out, looking at the Sun can produce immediate, localized damage to the retina, but not total blindness delayed many years. As in the case of Galileo, the actual details are completely inconsistent with any relation between the youthful solar work and blindness in old age.
One should remember that many people become blind in old age; cataracts, glaucoma, diabetes and other infirmities are still with us, and took a greater toll in past centuries when no effective treatments existed. A long list of prominent people who died blind could be adduced: Newton's contemporaries James Gregory and Leonhard Euler, the musicians Georg Friedrich Handel and Johann Sebastian Bach, the artists Piero della Francesca and Claude Monet, the modern writers Jorge Luis Borges and Jean-Paul Sartre, the biologist Jean-Baptiste Lamarck, and even King George III of England all became blind — but not from looking at the Sun. Indeed, F. C. Blodi has compiled a list of 56 famous people with vision problems.
Just as in the case of Galileo, one occasionally finds other famous people who became blind, and are falsely reported to have done so by looking at the Sun. A good example is Joseph Antoine Ferdinand Plateau, inventor of the stroboscope and other ingenious physical apparatus. He did a thesis on afterimages and colors, and one often finds claims (such as the one at St. Andrews) that his blindness later in life was from staring at the Sun for 25 seconds.
There is an extensive website at the University of Ghent devoted to Plateau, where a separate page is devoted to Plateau's blindness. There, one reads:
In many (popular) publications the blindness of Plateau is ascribed to his experiment of 1829 in which he looked directly into the sun for 25 seconds. Recent research definitely refutes this.
They don't offer a reference, but it is not difficult to find:
J. J. De Laey
De blindheid van Joseph Plateau. Mythe en realiteit
Tijdschrift voor Geneeskunde 58, 915–920 (2002)
The English abstract says, in part:
Plateau's visual problems apparently started when he was 42 years old. He considered them to be the consequence of an experiment in 1829 when he gazed for more than 25 seconds directly in the sun. However, we have reasons to believe that his blindness was due to chronic uveitis, in a period where there was no efficient treatment of this condition.
Apparently, Plateau suffered a temporary scotoma from his sun-gazing, but recovered from it in a few days. His later blindness had other causes; to connect it with a temporary impairment some 13 years earlier is to commit the logical fallacy known as post hoc ergo propter hoc.
It is often hard to say how such stories arise. But in Galileo's case, Rigaud (cited above) provides the explanation. On p. 33 of his Supplement, he says that the astronomer Joseph Jérôme de Lalande (1732–1807)
quotes from Scheiner, that the first inventor of telescopes lost his life from an inflammation of the eye, brought on by looking at the sun; he then mentions the blindness which came upon Galileo and Cassini in their old age, and warns astronomers against the fatal negligence of necessary precautions in observing.
(citing Lalande's Astronomie, § 2474). But even Rigaud is skeptical; he adds:
But it must have been something very different from negligence which could have operated so widely, and upon such men as we have just mentioned.
Indeed, if you follow Lalande's reference to Scheiner (Rosa Ursina, p. 69), it turns out that even Scheiner doesn't present his improbable story as fact, but as rumor or hearsay: his sentence ends with “narratur”. So it appears that the blame must fall primarily on Lalande for making up, or at least, widely promulgating, this fairy tale of astronomers who became blind from looking at the Sun.
Perhaps it is understandable, as the causes of blindness in old age were unknown in the 18th Century. (Lalande even cautions that “it is essential … not to look at the Moon for a long time … .”) But now, over two centuries later, it is time to put a stop to this nonsense.
After writing much of the above, I received a first-hand modern account of accidentally looking through a small telescope at the Sun, from William Bunker, who says:
Thanks for the reprint on retinal bleaching. Now I have something else to think about when observing the sun!
I have my own unfortunate example of retinal overload which occurred about 15 years ago, shortly after I became the proud owner of a 3 1/2 " Questar....I pass this along not as an example of my carelessness/stupidity, but as a warning to others who might repeat it.
Observing the sun via the full-aperture solar filter from the patio in front of my house, I attracted the attention of a landscaper who was working across the street. When he walked over to see what I was doing, I showed him the sunspots and he asked a few questions about the scope. How close will it focus? I removed the solar filter and focused on some flowers about 25 feet away. We are both chatty types, and we continued to talk aimlessly for 15 or 20 minutes. He left and I turned my attention again to the sun.
Having learned that the easiest way to find the sun was to observe the shadow of the telescope and minimize the shadow of the OTA, I did just that. Then I put my eye up to the eyepiece and observed the 6X image via the finder (which has its own solar filter). Then, without moving my eye, I flipped the mirror back into the main optical path. I jumped back from the telescope as if I had been stabbed in the eye, and suffered the sensation of disbelief common to those who have used a table saw for years, only to one day cut off a few fingers. I had forgotten to replace the main solar filter. For over thirty minutes there was a bright disc in the center of vision of my right eye. As I recall it now, this was green, but maybe I recall it that way because that's what I would have predicted from my limited knowledge of visual physiology. I escaped major damage, but I believe the acuity in that eye is reduced when compared to the left. The fact that the telescope was still focused for 25 feet and wasn't precisely centered may have saved me. I'll never forget the moment...I thought I had had a glimpse of hell.
I wrote back, pointing out that the major hazard in this situation is the danger of a thermal burn to the iris, rather than injury to the retina. The extreme bleach his retina suffered in a fraction of a second should have evoked Cornsweet's phenomenon; and this seems to be confirmed by the green appearance he remembers. The time scale of half an hour is also typical for recovery from a retinal bleach; cf. the similar time scale reported by Verschuur in my JOSA paper on retinal bleaching.
I also mentioned that the slight defocus would not have appreciably altered the surface brightness of the solar image. So, even though this was a narrow escape, the lack of permanent damage shows that the retina can, indeed, just tolerate a very brief exposure to the image of the Sun. He replied:
I remember the sensation of an intense blast of heat along with the overwhelming brightness. Thanks for the additional information on the likely damage (or lack of same) in this exposure. I have read many warnings against using solar filters at the eyepiece because of the intense heat and danger of fracture. My 12.5" Newtonian reflector even has a big metallic sticker warning against looking in the direction of the sun...although I have looked close enough to see Venus in the daytime. Mercury would make me a bit nervous!
Of course you have my permission to use my story; edit it or rearrange it as you like. If you wish, you may use my name and email address. I consider myself to be very careful and cautious in my dealings with potential injury, and have been even as a child. I have often weighed the unlikely risk of serious damage against the slight reward of an adrenaline rush, and voted to abstain from risk-taking. That's why this bit of carelessness so surprised me. Now when observing the sun, I always put my hand near the eyepiece before looking in!
Copyright © 2001 – 2003, 2005, 2006, 2012 Andrew T. Young