Oct. 1 8, 1883] 



NA TURE 



601 



panding itself into the delicate layer of nervous elements, lit, 

 which lines the inner surface of the vitreous cavity. 



When any object is seen by the eye, the rays of light emanating 

 from that body, impinging first upon the curved corneal surface, 

 have to pass successively through Aq, Cry, and Vt, before they 

 can affect the nervous retinal elements and cause the sensation of \ 

 light. In passing through these portions of the eye, the rays of 

 light are dealt with in a peculiar manner, especially perhaps by 

 the crystalline lens, and are brought together to form what is 

 c died an image on the retina. This image influences the nervous 

 elements of which the retina is composed in such a way, that a 

 sort of telegram is sent to the brain through the optic nerve, and 

 the brain becomes conscious of having seen something, the par- 



ticular object seen being included in the message. Another 

 diagram (Fig. 12) will perhaps make it a little clearer how this 

 image on the retina is formed. At A B is an arrow ; from it 

 rays of light are marked going to the three different points on' 

 the retina. But it will be seen that those rays of light which 

 come from the top of the arrow are, by the action of these three 

 media, twisted downwards, and form an image of the top por- 

 tion of the arrow on a low part of the retina. The rays of light 

 proceeding from the bottom of the arrow are bent up, so that its 

 image is formed on an upper part of the retina. The light 

 coming from the middle of the arrow is not bent at all, and 

 therefore forms its image on a middle portion of the retina. 

 That is the way in which the eye deals with rays of light entering 



Fig. 12.— i. Diagram showing path of rays when viewing an object at an easy distance. 1. Object brought close to eye when the lens L is required 



to assist the eye-lens to observe the image when the object is magnified. 



it. With this knowledge of the optics of the eye, it will be very 

 easily seen how very wonderfully the construction of the eye has 

 been imitated in a photographic camera. The front lens is 

 practically the equivalent of those three refractive media of the 

 eye, the aqueous and vitreous humours, and the crystalline lens ; 

 whilst the iris, which in the eye serves to limit the amount of 

 light entering it, has its exact representative in the "stop," 

 which serves the same end in the camera. The photographic 

 plate is, it need hardly be said, the counterpart of the re- 

 tina, and has consequently been beautifully described as "a 

 retina which does not forget." Similarly there is just such an 

 arrangement for focusing the light as exists in the eye. In 

 fact a camera is a rather better machine altogether than the eye, 

 because the range is greater, and the focusing power is not lost 

 as age increases. Therefore the artificial eyes of our camera 

 are never in need of spectacles. 



1. How Optics enables us to Read Fine Verniers. — This know- 

 ledge, then, having been acquired, how is it to be utilised for 

 the purpose of the measurement of angular space ? It may be 

 utilised in this way. The reason that we cannot clearly distin- 

 guish objects placed very close t) the eye is, that the rays of 

 light which flow from them are so extremely divergent that the 

 crystalline lens cannot focus them on the retina. But by placing 

 between the eye and the object a double convex lens, that is a 

 lens like the crystalline lens of the eye, this extreme diver- 

 gence is corrected ; the crystalline lens is thus aided, and the 

 rays of light are brought to a focus, as shown in the lower 

 part of Fig. 12. Take the case of a vernier whose divi- 

 sions are so fine that they are not visible at the distance of 

 distinct vision, say about ten inches. If we attempt to correct 

 this by making the divisions appear larger, by bringing the 

 vernier close to the eye, we lose the power of focusing the rays 

 which flow from it. But the introduction of a convex lens 

 between the vernier and the eye enables the eye to see the 

 division quite distinctly. 



Of course the more nearly an object approaches the eye, 

 the more powerful must be the lens, in order that the eye 

 may clearly see it. In this way we see that the simple addition 

 of a convex lens has enormously increased our power of ob- 

 serving and measuring small angles. 



2. One can, however, go further than this, and use not one 

 simple lens, but a combination of lenses. But before discussing 

 the various combinations of lenses which are employed in various 



instruments, it is necessary to look a little more closely than we 

 have yet done at the structure and action of our convex lens. 

 Let us use a glass lens in conjunction with an electric lamp. 

 Then we may get an image of the carbon poles thrown on the 



Fig. 13.— Formation of a lens from sections of prisms. 



screen, in exactly the same way that the crystalline lens forms its 

 image on the retina. But there would be this important differ- 

 ence, that while the image formed by the crystalline lens would 



