SENSE ORGANS. 



H3 



Experiment i. — Hold up the finger against the op- 

 posite wall or against the sky, and look not at the finger 

 but at the wall or sky. Two fingers are seen, shadowy, 

 transparent, because they hide nothing; the place cov- 

 ered by each is seen by the other eye. While still look- 

 ing at the sky or wall, shut the right eye; the left image 

 disappears. Shut the left eye ; the right image disappears. 

 Evidently the right image belongs to the left eye and 

 the left image to the right eye. Such are called heter- 

 onymously double images. 



Experiment 2. — Hold the two forefingers, one be- 

 fore the other, directly in front — i. e., in the middle plane 

 of the head, and twelve to fifteen inches apart. Look 

 at the farther finger; the nearer one is double. Look at 

 the nearer finger ; the farther one is double. By shutting 

 alternately first one eye and then the other it will be found 

 that in the former case the images each belong to the 

 eye on the opposite side — i. e., are heteronymous, while in 

 the latter case they belong each respectively to the eye 

 on the same side. Such are called homonymous. 



We might multiply experiments indefinitely, but these 

 are sufficient to show that we often see objects double. 

 They show more, viz., that when we look at an object 

 we see it single, but all objects beyond or this side of 

 the point of sight are doubled, but in opposite ways — 

 in the former case homonymously, in the latter heter- 

 onymously. This doubling of objects is evidently the 

 necessary result of the two retinal images. But the 

 questions occur: Why should we see objects single at 

 all ? What are the positions of the two retinal images 

 when objects are seen single ? 



Single Vision. — Since there are two retinal images 

 of every object and two external images, their spatial 

 representatives, it is evident that single vision can only 

 take place when the two external images are superposed and 



