CHAPTER III. 



SIGHT. 



SEC. 1. ON THE GENERAL STRUCTURE OF THE EYE, 

 AND ON THE FORMATION OF THE RETINAL IMAGE. 



§ 524. In dealing with the brain we have been incidentally 

 obliged to deal with some of the facts connected with the senses ; 

 but we must now study the details of the subject. And, for the 

 very reason that it is the most highly developed and differenti- 

 ated sense, it will be convenient to begin with the sense of 

 sight ; we shall find that the study of it throws more light on 

 the simpler and more obscure senses than the study of them 

 throws on it. 



A ray of light entering the eye and falling on the retina 

 gives rise to what we call a sensation of light ; but in order that 

 distinct vision of any object emitting or reflecting rays of light 

 may be gained, an image of the object must be formed on the 

 retina, and the better defined the image the more distinct will 

 be the vision. Hence in studying the physiology of vision, our 

 first duty is to examine into the arrangements by which the for- 

 mation of a satisfactory image on the retina is effected ; these 

 we may call briefly the dioptric mechanisms. We shall then 

 have to inquire into the laws according to which rays of light 

 impinging on the retina give rise to nervous impulses, and into 

 the laws according to which the sensory impulses thus gene- 

 rated, which we will call visual impulses, give rise in turn to 

 visual sensations. Here we shall come upon the difficulty of 

 distinguishing between the events which are of physical origin, 

 due to changes in the retina and optic fibres, and those which 

 are of psychical origin, due to features of our own consciousness; 

 for many of our conclusions are based on an appeal to conscious- 

 ness. We shall find our difficulties further increased by the fact, 

 that in appealing to our own consciousness we are apt to fall 

 into error by failing to distinguish between those affections of 



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