SS ee ee ee a ee ae 
J. LeConte on Binocular Vision. 8 
ments of Fedderson in 1858, and of Prof. O. N. Rood in 1869, 
give nearly the same results; the former ‘00004 of a second, 
and the latter from ‘000022 to -000050* depending upon the 
. . 
a second. Now it is obviously impossible that in s;}55 OF 
even in <=}, of a second the eye can change its convergence so 
as to adapt it consecutively to single visions of different objects 
at different distances. The perception of stereoscopic relief 
under these circumstances is therefore inexplicable on any other 
theory than that which I propose. The ¢rue theory of binocular 
perspective seems, therefore, to be this: the eye, even when fixed 
steadily on one point, perceives the relative distance of objects by 
means of double images, as already explained ; but this perception 
is made much clearer by the ranging of the eyes back and forth, 
uniting successively the images of near and distant objects. 
e pictures on a stereoscopic card be reversed, i. e. the 
right picture placed on the left side and the left picture on the 
right side, the binocular perspective is also reversed, the objects in 
the foregrounds being seen at a distance, and objects in_the 
backgrounds near at hand; in other words, the foregrounds of 
grounds of the pictures the foreground of the scene. The rea- 
son is obvious. By changing the pictures, identical points im 
the backgrounds become nearer together than those of the fore- 
ind, where aa and bd are seen. Fig. 1 represents the result 
where the pictures are properly mounted, and fig. 2 when 
reversed. By comparing the two figures the reverse perspective 
and its cause becomes evident. 
This inverse perspective was long ago pointed out and ex- 
plained by Wheatstone, and stereoscopic pictures are often made 
expressly to exhibit it. I am not aware, however, that any one 
has drawn attention to the beautiful, and in some respects 
another—the time between the occwrrence of the sparks and not the duration of the 
j t of the image of middle spark, 
I am indebted to my 
* This Jour., I, vol. xlviii, p. 153. + This Jour., III, vol. i, p. 15. 
} The italic a and 6 are underlined in the figures. 
