286 NATURE'S TEACHINGS. 



This may be easily ascertained by looking at a given object 

 first with one eye, and then with the other, when it will be seen 

 that the image presented to the right eye is slightly different 

 from that of the left eye, but that the two can be combined into 

 one by a very slight inward movement of both eyes, and thus 

 the effect of a solid Jbody be produced. Sometimes, when people 

 are weak, and cannot control the united movement of the eyes, 

 not only two, but five or six images are at once presented to 

 the mind, and produce a strange sense of bewilderment and 

 confusion. 



COMBINATION OP HUMAN EYES. STEREOSCOPE. BINOCULAR MICROSCOPE. 



Painters are obliged to avail themselves of this peculiarity, 

 and to make allowances for the double vision. If they do not, 

 the effect of the painting is flat, and it appears as if the artist 

 had only used one eye. 



A good proof of this fact may be seen in Stereoscopic photo- 

 graphs, especially of scenery. If each be viewed separately, 

 it often appears quite unintelligible, but, when they are com- 

 bined by the instrument, they seem to spring into life as it 

 were, and appear solid enough to be grasped. 



Now, the Stereoscope is avowedly constructed on the same 

 principle as the double vision of the eye, so that when it applies 

 itself to two photographs of the same object which have been 

 taken from different points of view, it combines them, and 

 gives them as solid an appearance as if they were realities. 



