160 Mr. A. A. Common [May 30, 



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 organi- 

 sation, produce the sense of vision. Possibly when these points are 

 affected 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 extends. Experiment tells us that it ex- 

 tends 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 remem- 

 ber 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 between 

 a circle, square, and triangle, can be detected when the linear size of 

 their images on the retina 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 moou. Taking the angular diameter of the 

 moon as 30 minutes of arc, 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 1/2U0 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 

 to 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 a 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 falling upon it. In order to make 

 this rather important point plainer, I have had a diagrammatic draw- 

 ing made on this plan. Taking a circle to represent the full moon 

 I have divided it into this number of spaces, and into each space 



