CHAPTER II. 

 SIGHT. 



A RAY of light falling on the retina gives rise to what we call a 

 sensation of light ; but in order that distinct vision of any object 

 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 sensory impulses, and those according to which 

 the impulses thus generated give rise in turn to sensations. Here 

 we shall come upon the difficulty of distinguishing between the 

 unconscious or physical and the conscious or psychical factors. 

 And we shall find our difficulties increased by the fact, that in 

 appealing to our own consciousness we are apt to fall into error by 

 confounding primary and direct sensations with states of conscious- 

 ness which are produced by the weaving of these primary sen- 

 sations with other operations of the central nervous system, or, 

 in familiar language, by confounding what we see with what we 

 think we see. These two things we will briefly distinguish as visual 

 sensations and visual judgments; and we shall find that both in 

 vision with one eye, but more especially in binocular vision, visual 

 judgments form a very large part of what we frequently speak of 

 as our sight. 



