SENSE ORGANS. 



149 



inal and spatial. We have just explained the correspond- 

 ing points of the two retinae. Now we assert that the 

 corresponding points in the two retince have the same spatial 

 correspondent. So that there is a kind of triangular 

 correspondence between the two eyes and space. 



The Two Fundamental Laws of Vision. — There 

 are also, as we have seen, two fundamental laws of 

 vision — the law of direction and the law of corresponding 

 points. The one explains the apparent anomaly of erect 

 vision with inverted retinal image, the other the appar- 

 ent anomaly of single vision with two retinal images. 

 The one is the fundamental law of monocular, the other 

 of binocular vision. We have seen how all the phenom- 

 ena of monocular vision flow logically from the one. 

 Now we proceed to show how all the phenomena of 

 binocular vision follow necessarily from the other. 

 There is, however, a third law underlying both and more 

 fundamental than either — viz., the law of outward or 

 spatial reference of all retinal states. 



BINOCULAR PERSPECTIVE. 



The law of external reference gives space. The law 

 of direction gives two dimensions of space — i. e., up and 

 down and from side to side. Now, the law of corre- 

 sponding points gives the third dimension of space — i. e., 

 depth or distance from the observer. The perception 

 of this third dimension, so far as it is dependent on the 

 use of the two eyes as one instrument, is our next sub- 

 ject. We begin again with experiments : 



Experiment i. — We repeat that given on page 143, 

 but for another purpose. Place the two forefingers, one 

 before the other, in the median plane, and separated, say, 

 a foot from one another. We have already shown that 

 when we look at the nearer finger we see it single, but 

 the farther finger is doubled homonymously. When we 



