344 SEA-SIDE STUDIES. 



the brain ; on tlie contrary, the thing transmitted is a 

 sensation, or group of sensations, excited by what is called 

 the " image." The wave of light is translated into a nerve- 

 stimulus, the impression excites a sensation ; but the sensa- 

 tion is due to the specific centre, not to the specific stimulus 

 of light ; as we know by the fact that any other stimulus, 

 such as pressure or electricity, is translated into a precisely 

 similar sensation. So that even if we suppose an image to 

 be formed on the retina, as it is formed in a camera-obscura, 

 it will not be transmitted to the brain, but it will excite the 

 specific sensations of which the oj)tic centre is alone capable, 

 and these will be transmitted.* 



When, a little while ago, I said tliat the formation of an 

 image was a primary requisite in vision, I meant that unless 

 the rays from an object converged into an image on a proper 

 surface, no distinct perception of that object could result. 

 The reader will not, therefore, suppose that, in throwing 

 doubt on the notion of images being formed on the retina, as 

 they are formed on the camera-obscura, any attempt will be 

 made to overthrow the optical principles minutely estab- 

 lished by philosophers. A brief description of the retina 

 and its connections will enable us to argue this point at 

 our ease.-|- 



The retina is not, as commonly supposed, simply an expan- 

 sion of the optic nerve — if by that be meant a purely fibrous 



* " Light and colour are actions of the retina, and of its nervous prolonga- 

 tions to the brain."— Mueller: Physiology, Eng. Trans., p. 1102. 



+ The student should carefully read KoLLIKEII, Ilundbuch der Gewehelehre, 

 and II. MiiLLER, A natomisch-physiclogische U nlersuchunyen iXber die Retina lei 

 Mtnsclun u W irbdthieren. 185(5. 



