Trans. N. V. Ac. Sci. 1 4 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 formulas 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 C) ; 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, he con- 

 structs a table of apparent distances corresponding to various optic 



(*) For description, see Phil. Mag., s. 4, vol. III., June, 1852, p. 506. 

 V) Phil. Mag., s. 4, vol. III., p. 504. 



