54 



• KNOWLEDGE 



[Junk 23, 1882. 



THE EYES OF SCIENCE. 



Bv THE Editor. 



THE U'lcscopo, tlie microscope, and tho spectroscope 

 give to the student of science what may be re- 

 garded as three kinds of visual power — in one case 

 unlike the visual qualities possessed by the natural 

 eye, in the other two surpassing these greatly in 

 degree. We can conceive creatures endowed with the 

 powers of vision which the telescope and the microscope 

 artificially supply. Indeed it is by no means incredible 

 that in other worlds than ours creatures may e.xist possess- 

 ing powers akin to these. And although it is not easy 

 to conceive the sense of vision so increased and extended 

 that by means of it the analysis of liglit effected artificially 

 by tlie spectroscope could be eQcctcd naturally, yet there is 

 nothing absolutely outside the range of possibilit}-, even in 

 this. The eye is, indeed, an optical instrument, precisely 

 as the ear is an acoustical instrument ; and so far as wc can 

 judge, the sense of vision might have been provided with a 

 more complex organ, or series of organs, giving it greater 

 range, as in the telescope, or more complete power of magni- 

 fying minute details, as in the microscope, or the power of 

 separating light-rays of diUerent refractive nature, as in the 

 spectroscope. There are other optical instruments also 

 ■whose powers might have had tlieir analogues in the organ 

 of sight (as the i olariscope and similar instruments) ; while 

 there are others, as the stereoscope and so forth, which, 

 like the telescope and microscope, are akin to the organ of 

 vision, but give to it increased power in particular way.s. 



I have lately been led to notice how certain photographic 

 processes and methods extend tlie powers of human vision, 

 and enable us to see what, owing to certain peculiarities 

 in the circumstances under which eyesight is employed, 

 we are debarred from seeing in the ordinary way. 



It has long been noticed that photographic vision, so to 

 describe this method of studying natural objects, has one 

 great advantage over ordinary vision in that it is not liable 

 to ordinary misleading influences. In science, seeing is 

 not always, or even generally, believing — for the simple 

 reason that the student of science cannot always be 

 certain what he really sees. 



Thus an observer may be misled by imagination, es- 

 pecially if some favourite theory has possession of his mind. 

 If he knows, or thinks he knows, what he ought to see, or 

 might fairly expect to see, he is very apt to imagine that 

 he actually does see it In this way, for instance, many 

 students of astronomy liave fancied they have seen a small 

 companion by a star in a position where they had been 

 told such a companion existed, when, in reality, there had 

 been some error in the description, or in their reading of 

 it, and either no such companion existed, or else it was in 

 some entirely different position, and perhaps quite beyond 

 tlie range of the telescope employed by the observer. 



Again, the eye is repeatedly deceived by effects of con- 

 tra.st. Thus, the French astronomer Chacornac advanced a 

 Tery ingenious— indeed, masterly— theory in explanation 

 of the circum.stance that the disc of the planet Jupiter is 

 brighter near tlie edge than in the niiddh;, tho only objec- 

 tion to his theory rcsidiug in the circumstance that the disc 

 is darker, not bright/r, near the edge, though to the eye it 

 appears brighter there by contrast with tho dark back- 

 ground of the sky on which it is 8ecn proj(;cted. 8o again 

 there is a charming theory, in vogue tfj this day among 

 many students of the n.oon, explaining why the floor of 

 the lunar crater I'lato (the Greater Black Lake of the 

 «rlier t<;lescopists) grows darker as the sun pours more 

 light U{Kjn it (rising higher in the sky as supposed to be 

 viewed from Plato), the real fact lx,iiig that there is no 



such darkening, the apparent dilforence being entirely due 

 to eiiVcts of contrast — the contrast of the floor with the 

 black shadows of the crater-ring thrown upon it wlicn the 

 sun is low, and the contrast of the floor with the brilliant 

 white of the surrounding crater-ring when the sun is high, 

 one contrast making the floor look lighter than it is, 

 wliile the other makes it look darker. I may cite 

 another instance of an optical illusion, caused by on 

 etiect of contrast— a case not requiring telescopic obser- 

 vation for its recognition. If on a moonlit night one 

 looks beyond a water horizon towards the part of tho sky 

 below the moon, that region looks darker than the parts 

 of the sky on either side ; yet, in reality, it is no darker 

 — if anything slightly lighter. What causes it to look 

 darker is the apparent* brightness of the part of the 

 water just below the moon, where lies seemingly* a broad 

 track of silver light. If this track of light is concealed 

 in any way, as by holding up a sheet of card or paper, 

 the portion of the sky immediately above is at once 

 seen to be at least as bright as the parts of the sky on 

 either side of it. So in multitudes of other cases — some 

 familiar, some otherwise — the eye is deluded by effects 

 of contrast. 



Photography, or what may be called photographic 

 vision, is not, it is true, altogether free from defects, 

 corresponding to such defects of vision (resulting in illu- 

 sion) as we have just considered. As tliere are physio- 

 logical illusions in ordinary vision in such cases, so are 

 there in certain applications of photography, physical 

 effects which may prove similarly illusive. For instance, 

 there is what is sometimes called photographic irradia- 

 tion, when around a dark object in a photograph a ring o£ 

 light is seen, or around a bright object a ring of darkness, 

 this ring not corresponding to any really existent object, but 

 resulting from some change in the photographic film along 

 the border-line around a region acted on very strongly by 

 light. 



Again, the photographic eye has long been justly valiuid 

 for its artistic power, in being able to record, without 

 defect or exaggeration, what it sees. If we take, for 

 instance, one of Dr. llutherfurd's photographs of the solar 

 disc, and compare the spots there depicted with those 

 shown even in the most carefully-executed pictures of the 

 sun before and since, we see at once how liable the eye is 

 either to be deceived in what it sees, or else to fail duly to 

 guide the hand in reproducing what the eye has seen. I 

 happen to know of a case where a draughtsman took ex- 

 ceptional pains to reproduce, without exaggeration, the 

 aspect of the solar disc with its spots, when yet, on com- 

 parison being made with a photograph taken nearly at the 

 same time (though 3,000 miles away), it appeared that the 

 spots had been notably exaggerated. I refer to tho draw- 

 ing of the sun's face which forms the frontispiece of my 

 treatise on The Sun. I was particularly struck by the 

 aspect of the sun when that drawing was made, and I 

 certainly spared no pains to delineate tho spots correctly ; 

 but a comparison of my picture with a well-known photo- 

 graph by Kuthurfurd, which chanced to be taken aliout the 

 same time in New York, will show that though the spots 

 are delineateil, individually, correctly enough, they are 

 consi<lcrably too large as compared with the, solar disc — an 

 enlargement by no means necessary to enhance their im- 

 portance, for the largest spot visible on that occasion had 

 a surface several times larger than the entire surface of 

 this earth. 



• I say "apparent" and "fiocmingly " because tho moon's rays 

 really illuminato the region which appoars dark, as brightly as tho 

 rent. It is only bocauHo of the position of tho obsorvei'a oyo that 

 one region aiipcara brighter than the rest. 



