CHAPTEE III. 

 SIGHT. 



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

 AND ON THE FORMATION OF THE RETINAL IMAGE. 



702. 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 differentiated 

 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 formation 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 generated, 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 consciousness. We shall find our difficulties further 

 increased by the fact, that in appealing to our own conscious- 



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