SENSE ORGANS. 151 



tween the two ends. The same is true of all objects 

 occupying depth of space. 



Thus, then, we may safely generalize : In viewing a 

 single object occupying considerable depth of space, or 

 a scene with objects one beyond the other, it is evident 

 that the retinal images of the object or of the scene in 

 the two eyes, and therefore the external images — their 

 spatial representatives — or the way the object or scene 

 looks to the two eyes, respectively, are different, because 

 taken from different points of view. Therefore they can 

 not be united as a whole, but only in parts at a time. 

 When we look at the foreground, objects in the back- 

 ground are double ; when we look at the background 

 objects in the foreground are double. Thus we run the 

 the point of sight back and forth, uniting successively 

 different parts of the scene, and acquire thus a clear 

 perception of depth of space between. 



Limitation of Clear Vision.— See, then, the ex- 

 treme limitation of distinct vision and of single vision. 

 As distinct vision is confined to a small area about the 

 point of sight, and we must therefore sweep about this 

 point and gather up the result in memory, even so 

 single vision is limited to the distance of the point of 

 sight, and we must run the point of sight back and forth, 

 uniting successively different parts of the scene, thus 

 probing space and gauging its depth, and gather up the 

 results in memory. 



Different Forms of Perspective.— Of course, there 

 are other ways of judging of relative distance — other 

 forms of perspective. It may be well, therefore, to give 

 these, and very briefly compare them : 



1. Aerial Perspective. — We judge of distance by the 

 color of the air through which we look. The atmos- 

 phere is not absolutely transparent, but bluish. Distant 

 objects, like mountains, are dimmer and bluer in pro- 



