Trans. N. Y. Ac. Sci. 10 Oct. 24, 
deviated by the semi-lenses, they appear to have come from A. In like 
manner, the background appears at B. If 7 = interocular distance 
RL, and a = optic angle, then for the distance of A we have 
D=+42zcorta 
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 A, and Az be removed to Mi 
and M: 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 
accustomed to analyzing his own visual sensations. Upon a uniform 
horizontal surface (Fig. 2) let two lines, A C and B C, be drawn, form- 
Fic. 2. 
ing a small angle, 8, with its vertex toward the observer. Let the eyes, 
