OCULAR IMAGE. 



This will be rendered more clear by reference to fig. 1. Let 

 A, B, c, be a candle, for example, placed before the eye, E. Rays 

 diverge from the top, A, of the flame, and enter the pupil. A cone 

 of these rays, whose point is at A, and whose base is the pupil, 

 enter the eye, and being collected on the retina, produce a percep- 

 tion of the point A.* And other cones, or PENCILS, as they are 

 called, proceeding from the points B and c, and, in general, from 

 all the points of the candle, radiate to the pupil in like manner, 

 and severally produce perceptions, and so a perception of the candle 

 is produced. 



Now, if A, B, c, instead of being a real candle, were merely the 

 optical image of a candle, the same perception of its presence 

 would be produced, provided the same rays radiated in like 

 manner from each point to the eye, and the observer would see 

 it exactly as he would see the object itself, were it in the same 

 position. 



But it is not even necessary to the production of the perception 

 that either the object or its image should be present, if the rays, 

 no matter where they may have originated, or what route they 

 may have followed, only enter the eye in the same lines of direc- 

 tion which they would have, had they come directly from the 

 object. Thus, for example, if the pencils, instead of coming from 

 A and c, had come from a similar point at A' and c' towards a and 

 c, and had there by any optical agency been turned into the direc- 

 tions which they would have had, if they had come from A and c 

 to the pupil, the perception produced by them would be exactly 

 the same. 



In fine, the perceptions produced depend on the directions 

 which the rays have in entering the pupil, and are altogether 

 independent of the route they may have followed before arriving 

 there. 



It will be most necessary that this fact be impressed on the 

 memory, since the whole theory of vision, especially where optical 

 agents are used, depends more or less upon it. 



3, IMAGES PRODUCED BY PLA^E REFLECTORS. 



The most simple case of the production of optical images, and 

 that of most frequent occurrence, is when they are produced by 

 reflection from plane surfaces ; as when a landscape and the firma- 

 ment are seen reflected in the surface of water, or when objects are 

 seen in a looking-glass. 



To explain this very familiar phenomenon, it is necessary first 



* See Tract on THE EYE, vol. v., pp. 54, 55. 



o 2 83 



