ii4 INTRODUCTION TO GENERAL PHYSIOLOGY 



light takes the important place that is attained by it. It is, how- 

 ever, very early in the scale of evolution that eyes are found. The 

 jelly fish possess them, although they do not possess the necessary 

 elaboration of structure required to form distinct images. The 

 perception of sound, contrary to that of light, seems to be a 

 comparatively recent development. 



The layer or coat at the back of the eye in which the nerve 

 fibres end is called the " retina " (E., p. 214). It contains a substance 

 called "visual purple," which is sensitive to light (E., p. 212). We 

 saw, in discussing the action of chlorophyll, that in order that light 

 energy should have any effect it must be absorbed, and the 

 magnitude of the effect is naturally in proportion to the amount 

 of light absorbed. Investigations of the properties of visual purple 

 have shown that its absorptive power for different parts of the 

 spectrum agrees with the sensibility of the retina to these parts. 

 Further, the effect of light in bleaching the pigment follows the 

 same course, and also does the apparent brightness of the different 

 parts of the spectrum. We are therefore justified in regarding this 

 pigment as the seat of the photo-chemical reaction at the basis 

 of vision (see P., p. 521). 



But a mere sensibility to light would be of comparatively 

 little value. It is necessary to have a means of producing a 

 picture of external objects on the retina, so that different parts 

 of this picture may stimulate separate nerve fibres, and a 

 representation of it be conveyed to the brain. 



A familiar method of producing a picture on a sensitive surface 

 is that of the photographic camera, and it will be instructive to 

 compare its essential parts with the corresponding parts of the 

 eye. The student should examine these parts in the eye of an 

 ox (E., p. 213). The sensitive plate, as we saw, is represented by the 

 retina, and on this an image is formed by means of a lens which 

 gives a real image. Such a lens consists of the convex surface 

 of a medium having a higher refractive index than air. The ray 

 from each point of an external object is bent towards the centre 

 of the lens when it strikes it, and in proportion to the distance from 

 the centre of the lens at which the ray enters. At a certain 

 distance behind the lens an image is produced. If the object 

 is distant, the focal plane, as it is called, is nearer to the lens than 

 if the object is at a less distance, and the size of the image is less, 

 the nearer it is to the lens (E., p. 163). That part of the eye which 

 takes the place of the lens of the camera is not, as might be 

 thought, that structure which is actually called the " lens " of the 

 eye. This plays a comparatively small part in the formation of 

 an image, but has another function, as will be seen presently. The 

 actual lens is the front clear spherical surface of the eye, known as 



