Trans. N. V. Ac. Sc/. 10 Oa. 24, 



deviated by the semi-lenses, tliey appear to have come from A. In like 

 manner, the background appears at B. If / = interocular distance 

 RL, and « = optic angle, then for the distance of A we have 



D = ^ i cof i a 



From this formula it is obvious that D ceases to have any positive 

 finite value when the visual lines cease to converge. 



If the semi-lenses be taken away, and Ai and A2 be removed to Mi 

 and M2 respectively, while the convergence of visual lines remains un- 

 changed, the images still appear at A and B. Wheatstone seems to 

 have been the first to show experimentally that the illusion of apparent 

 solidity can be obtained in this manner from a pair of projections repre- 

 senting the same object from slightly different points of view. If the 

 eyes be properly trained, the visual lines may be directed to points 

 whose distance is greater or less than that of the objects regarded at 

 the same moment, and Brewster described many striking illusions thus 

 obtained without the aid of the stereoscope. The principle applied by 

 him, as described in the paper to which reference has been made, may 

 be briefly given, and his results can be easily tested by any one who is 

 liccustomed to analyzing his own visual sensations. Upon a uniform 

 horizontal surface (Fig. 2) let two lines, A C and B C, be drawn, form- 



i 



ti 





• -in- / 



II 



-i" 



Fig. 2. 



Jng a small angle, /3, with its vertex toward the observer. Let the eyes. 



