i84 



NA TURE 



[June 19, 1890 



shall assume that the eye sees all things best at the distance of 

 about nine inches, we may say that the picture taken with a lens 

 of this focal length gives at once the proper and most natural 

 representation we can possibly have of anything at which we 

 can look. Such a picture of a landscape, if placed before the 

 eye at the distance of nine inches, would exactly cover the real 

 landscape point for point all over. A picture taken wiih a lens 

 of shorter focal length, say four inches, will give a picture as 

 true in all the details as the larger one, but if this picture is 

 looked at, at nine inches distance, it is not a true representation 

 of what we see ; in order to make it so, we must look at it with 

 a lens or magnifier. With a larger picture one can look at this 

 at the proper distance, which always is the focal distance of the 

 lens with which it was obtained, when we will see everything in 

 the natural angular position that we have in the first case. 



But if, instead of looking at this larger picture, which we may 

 consider taken with a lens of say ninety inches focal length, at 

 a distance of ninety inches, we look at it at a distance of nine 

 inches, we have practically destroyed it as a picture by reducing 

 the distance at which we are viewing it, and we have converted 

 it into what is for that particular landscape a telescopic picture ; 

 we see it, not from the point at which it was taken, but just as if 

 we were at one-tenth of the distance from the particular part that 

 we examine. A telescope with a magnifying power of ten, 

 would enable us to see the landscape just as we see it in the 

 photograph, when we examine it in the way I have mentioned. 



Having thus seen how a lens or mirror acts, we will turn our 

 attention to the eye. Here we find an optical combination of 

 lenses that act together in the same way as the single convex 

 lens of which we have been speaking. We will call this com- 

 bination the lens ( f the eye. It produces a picture of distant 

 objects which in the normal eye falls exactly in focus upon the 

 retina. We are conscious that we do see clearly at all distances 

 beyond about nine inches. 



At less than this distance objects become more and more 

 indistinct as they are brought nearer to the eye. From v\ hat we 

 have seen of the action of the lens in producing pictures of 

 near and distant objects, we know that some movement of the 

 screen must be made in order to get such pictures sharply 

 focussed, a state of things necessary to perfect vision. We might 

 therefore suppose that the eye did so operate by increasing when 

 necessary the distance between the lens and retim, but we 

 know that the same effect is produced in another way ; in fact, 

 the only other way. The eye by a marvellous provision of 

 nature, secures the distinctness of the picture on the retina of all 

 objects beyond a distance of about 9 inches, by slightly but 

 sufficiently varying the curvature of one of the lenses ; by an effort 

 of will, we can make the accommodating power of the eye 

 slightly greater, and so see things clearly a little nearer ; but at 

 about the distance of 9 inches, the normal eye is unconscious of 

 any effort in thus accommodating itself to different distances. 

 The picture produced by the lens of the eye whose focal length 

 we will assume to be six-tenths of an inch falls on the retina, which 

 we will assume further to be formed of a great number of 

 separate sensible points, which, as it were, pick up the picture 

 where it falls on these points, and through the nervous organiza- 

 tion, produce the sense of vision. Possibly when these points 

 are alfected by light, there may be some connective action, either 

 produced by some slight spherical aberration of the lens or 

 otherwise ; but I do not wish to go any further in this matter 

 than is necessary to elucidate my subject. What I am concerned 

 with now is the extent to which the sensibility of the retina ex 

 tends. Experiment tells us that it extends to the perception of 

 two separate points of light whose angular distance apart is 

 one minute of arc, or in other words at the distance we can see 

 best, two points whose distance apart is about 1/400 of an inch. 



This marvellous power can be better appreciated when we re- 

 member that the actual linear distance apart of two such points 

 on the retina is just a little more than 1/6000 of an inch. 



In dealing with the shape of small objects the difference be- 

 tween a circle, square, and triangle, can be detected when the 

 linear size of either is about 1/2000 of an inch. It may be 

 therefore fairly taken that these separate sensible points of the 

 retina are somewhere about 1/12,000 part of an inch apart 

 from each other. Wonderfully minute as must this structure be, 

 we must remember, as we have already shown, that the actual 

 size of the image it deals with is also extremely small. This 

 minuteness becomes apparent when we consider what occurs 

 when we look at some well-known object, such as the full moon. 

 Taking the angular diameter of the moon as 30 minutes of arc, 



NO. 1077, VOL. 42] 



and the focal length of the eye at six-tenths of an inch, we find the 

 linear diameter of the picture of the full moon on the retina is 

 about 1/200 of an inch, and assuming that our number of the 

 points in the retina is correct, it follows that the moon is subject 

 to the scrutiny of 2800 of these points, each capable of dealing 

 with the portion of the picture that falls upon it. 



That is to say, the picture, as the retina deals with it, is made 

 up of this number of separate parts, and is incapable of further 

 division, just as if it were a mosaic. I think this is really the 

 case, and as such a supposition permits us to explain not only 

 what occurs when we assist the eye by means of the telescope, 

 but also what occurs when we use the telescope for photographing 

 celestial objects, we will follow it up. 



In the case of the eye we suppose the image of the moon to 

 be made up through the agency of these 2800 points, each one 

 capable of noting a variation in the light fallitig upon it. In 

 order to make this rather important point plainer, I have had a 

 diagrammatic drawing made on this plan. Taking a circle to re- 

 present the full moon, I have divided it into this number of 

 spaces, and into each space I have put a black dot, large or 

 small, according to the intensity of the light falling on that part 

 of the image as determined by looking at a photograph of the 

 moon. You will see by the picture of this moon the effect pro- 

 duced. It represents to those who are at a sufficient distance 

 the moon much as it is really seen in the sky. 



We can now with a lens of the. same focal length as the eye 

 obtain a picture of the full moon exactly of the size of the actual 

 picture on the retina, and if we take a proper photographic pro- 

 cess we can get particles of silver approximately of the same 

 sizes as the dots we have used in making our diagram of the 

 moon ; the grouping is not exactly the same, but we may take 

 it as precisely so for our purpose. I have not any photographs 

 of the full moon of this size, but I have some here of the moon 

 about five, seven, and eight days old, which give a good idea 

 of what I mean by the arrangements of the particles of silver 

 being like our diagram. 



It is now quite apparent that if we can by any means increase 

 the size of the picture of the moon on the retina or make it 

 larger on the photographic plate, we would be able to employ 

 more of our points or particles of silver, and so be able to see 

 more clearly just in proportion as we increase the size of the 

 picture in relation to the size of the separate parts that 

 make it. 



Now the telescope enables us to do this for the eye, and 

 a longer focussed lens will give us a larger photographic 

 picture. 



Let us assume that by means of the telescope we have in- 

 creased the power of the eye one hundred times. The picture 

 of the moon on the retina would now be one-half inch diameter, 

 and instead of employing 2800 points to determine its shape, 

 and the various markings upon it, we should be employing 

 28,000,000 of these points ; and similarly with the photograph, 

 by increasing the size of our lens we will obtain a picture made 

 up of this enormous number of particles of silver. But we can 

 go further in the magnification of the picture on the retina — we 

 can also use a still longer focus photographic lens. 



A power of magnification of one thousand is quite possible 

 under favourable circumstances ; this means that the picture of 

 one two-hundredth of an inch would be now of five inches in 

 diameter, so we must deal with only a portion of it. Let us 

 take a circle of one-tenth of this, equalling one-hundredth of our 

 original picture, which in the eye, unaided by the telescope, 

 would have a diameter of one two-thousandth of an inch, or an 

 area of less than one five-millionth of a square inch. This means 

 that with this magnification, we have increased the power so 

 enormously that we are now employing for the photographic 

 picture two thousand eight hundred million particles of silver, 

 and in the eye the same degree of increase in the number of 

 points of the retina employed in scrutinizing the picture piece 

 by piece as successive portions are brought into the central 

 part. 



Photography enables me to show that the result I have 

 given of the wonderful effect of increasing the optical power is 

 perfectly correct as far as it is concerned. We will deal with a 

 part only of the "moon, representing, as I have just said, about 

 one-tenth of its diameter, or one-hundredth of its visible sur- 

 face. Two such portions of the moon are marked, as you see, 

 on the diagram. I have selected these portions as I am able 

 to show you them just as taken on a large scale by photography 

 so that you can make the comparison in the most certain manner ; 



