66 



SPECIAL SENSES. 



observers. If in a card (fig. 42, * *) two small holes be pricked, 

 over or to the side of one another, but not more distant than the 



diameter of the pupil, A, 

 B, and a small object, such 

 as a pin, be looked at 

 through them, it will be 

 seen single only when it is 

 at a certain distance from 

 the eye, say at a ; for the 

 rays of the pencil which 

 proceeds from the object at 

 c, come precisely to a focus 

 upon the retina, at c. If 

 the pin be now placed at 

 b, the rays will centre at 

 g, in front of the retina, and 

 the object be then seen 

 double at d and f. The 

 same thing happens when 

 the pin is removed to a 

 greater distance than a, 

 say to e; the pencil of rays 

 in this case could only cen- 

 tre after their refraction by 

 the lens at h, far beyond 

 the retina, so that the sin- 

 gle object is necessarily 

 again seen double at i and 

 k. Double vision of this 

 kind sometimes occurs 

 along with partial opaci- 

 ties, streaks and specks of 

 the cornea. 



[ 135. Although there 

 are two images formed by 

 the refracting media upon 

 the retina of the two eyes, 

 still in ordinary vision we 

 see objects single, not double. This depends on the condi- 

 tion or quality of particular spots of the two retinae. Ob- 

 jects, to wit, are seen single when the axes of the two eyes 

 meet in the object contemplated. In this case the point fixed 



