

SENSE OF SIGHT. 



429 



the fact that, when we look at, the luminous point a, through 

 the two openings Mm and Nn (Fig. 115), a phenomenon of 

 adaptation takes place in the eye, by means of which the 

 summit of the ocular cone falls upon the retina; the summits 

 of the two partial cones passing through the two openings 

 are joined together in one (in a), these two cones making a 

 part of the total cone which would be produced if the lumi- 

 nous point were looked at with the naked eye ; this is the case, 



Fig. 115. Experiment by Scheiner.* 



however, only with regard to the point a ; the objective cone 

 of the point 5, being longer, its ocular cone will be shorter; 

 and the summit, consequently, will be in front of the retina, 

 and can strike this membrane only by diverging after bring- 

 ing about the intersection of its rays : if then, as in this ex- 

 periment, the cone be divided in two by looking through two 

 holes, the object 5, which is not looked at, will be projected as 

 two distinct cones (and will be seen double) ; since they strike 

 the retina, not at the point of their common summit (#'), but 

 farther back, where they are again separated (", b"). The 

 eye, in this case, is evidently so adapted as to see a and not 

 b : but if b be looked at attentively, cc, in its turn, will appeal- 

 double. 



These facts are sufficient to prove that we possess the 

 power of adapting or accommodating our sight to different 

 distances, and this is true up to a certain point, whatever be 

 the distance; thus we can see objects which are placed at an 



* A B, Diaphragm with two apertures (Mm and Nn). 



o, Point for which the eye is adapted, and the image of which appears at a 

 (on the retina). . 



b. Point for which the eye is not adapted ; the luminous rays proceeding from 

 it meet at V (in front of the retina), diverge again, and meet the retina at 6" 6", 

 causing the point b to appear double. 



