Trans. N. Y. Ac. Scz. 1 Oct. 24, 
and to the projection of images viewed in the stereoscope when the 
convergence of visual lines is identical with that of the camera axes, 
but not otherwise. Instead of human eyes we may assume a pair of 
camera lenses, an interocular distance apart, and a pair of sensitized 
plates behind them. Helmholtz’s tormulas enable us to determine the 
stereoscopic displacements in the images projected. If proofs from 
the negatives thus obtained be inverted and placed in front of a pair 
of eyes in such manner that the visual lines passing through corres- 
ponding photograph points shall bear to each other the exact relation 
that existed between the secondary camera axes that terminated in 
them, these two points will appear as one, and nearly at the distance 
of the real point in space to which the camera axes were converged. 
The effect is much the same as if the eyes, with normal convergence 
of visual lines, had been substituted for the cameras. But if the proofs 
be too near together or too far apart, increase of convergence makes the 
whole picture seem nearer, while divergence makes it farther. The 
relation between the different parts having been fixed at the time the 
picture was taken, increased convergence makes the distance from back- 
ground to foreground seem less, divergence makes it greater. No one 
can have failed to notice the gross exaggeration of perspective often 
seen in the stereoscope, when the pictures are so far apart as to make 
the visual lines parallel or divergent, while the angle between the 
camera axes, when they were taken, was relatively large. But in no 
case do these conditions cause variations of such magnitude as Brew- 
ster’s theory of binocular perspective would demand. This is easily 
illustrated with Wheatstone’s reflecting stereoscope.(*) Suppose the 
stereograph to represent a concave surface with the opening toward the 
observer, and that the arms of the instrument are properly adjusted. 
If they are pushed back, so as to make the visual lines divergent, the 
cavity apparently recedes and deepens; if pulled forward, so as to make 
them strongly convergent, it seems to approach and grow shallow. 
The apparent diameter of the image enlarges in the first case and 
diminishes in the second. Wheatstone notices this last variation in the 
account which he gave of his invention and its applications, in 1852, in 
the Bakerian lecture before the Royal Society (*); but, strange to say, 
the variation which is produced in apparent distance and depth under 
the same conditions seems to have escaped his notice, and the pos- 
sibility of using his instrument to test the peculiarities of binocular 
vision with divergence of visual lines, seems not to have occurred to 
him. For the refracting stereoscope, however, like Brewster, be con- 
structs a table of apparent distances corresponding to various optic 
(®) For description, see Phil. Mag., s. 4, vol. III., June, 1852, p. 506. 
(7) Phil. Mag., s. 4, vol. III., p. 504. 
