EFFECTS EXPLAINED. 



right side of the bust, which will be concealed from the left eye 

 by the projecting parts of the face. 



It therefore appears that the two eyes, right and left, will have 

 different views of the bust ; so that if the observer were to make 

 an exact drawing of the bust with his left eye closed, and another 

 exact drawing of it with his right eye closed, these drawings 

 would not be identical. One of them would show a part of the 

 bust on the extreme right, which would not be exhibited in the 

 other, and the latter would show a part on the extreme left, which 

 would not be included in the former. Moreover, a part of the 

 cheek and the eye would be shown in the drawing made with the 

 right eye closed, which would not appear in the drawing made 

 with the left eye closed. 



Two such views of the same object are shown in figs. 1 and 2, 

 the former being the view presented to the left and the latter to 

 the right eye. 



Now it is evident that when such an object is looked at with 

 both eyes open, the two different visual impressions here described 

 are simultaneously perceived, and they become to the mind signs 

 and indications of the actual forms which produce them. 



When objects, therefore, can be viewed at distances small 

 enough to be attended with a sensible degree of parallax, their 

 perspective and relief are perceived, not only by the outlines and 

 varieties of light and shade, which are the common indications of 

 perspective and relief at all distances, but also by the class of 

 binocular phenomena which we have just described. 



Hence it follows that the perception of relief, and generally of 

 form and relative position in objects whose proximity is sufficient 

 to produce binocular parallax, is much stronger and more vivid 

 than those whose distances, rendering the binocular parallax 

 evanescent, leaves nothing but the outlines and the varieties of 

 light and shadow, by which the mind can form a judgment of 

 form, relative distance, and position. 



But since binocular parallax is reduced to the very small 

 amount of half a degree at the distance of 24 feet, it is clear that 

 it can only enter into the conditions by which we perceive 

 perspective and relief, in the case of a very limited class of 

 objects, and is not at all applicable to objects in general whose 

 forms and perspective we habitually contemplate. 



5. After what has been explained of the two different views 

 which a near object presents, when looked at successively with 

 the one eye and the other closed, the principle of the stereoscope 

 will be easily understood. 



A bust being placed before a competent draughtsman, as above 

 described, at a distance sufficiently small to produce considerable 



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