eS ee ee ee 
AMERICAN 
JOURNAL OF SCIENCE AND ARTS, 
[THIRD SERIES] 
Art. L—On some phenomena of Binocular Vision ;* by JOSEPH 
LeConts, Prof of Geol. and Nat. Hist., Univ. of California. 
V. Stereoscopie phenomena. 
Ir isa familiar fact that in stereoscopic pictures, properly 
mounted, identical points in the foregrounds of the two pictures 
are always a little nearer together than identical points in the 
backgrounds. With a pair of compasses we can, by this means, 
easily test whether or not pictures are properly mounted. i 
evident therefore that it requires greater optic convergence to 
unite the foregrounds than the backgrounds of the two pict- 
ures, It is also evident that we cannot at the same time and 
Il respects to natural vigion of near and distant objects, instine- 
tively introduces the idea of depth of space. Or even looking 
steadily at any point, say in the middle ground, the depth of 
space is still perceived, as in nature under similar circumstances, 
for the same reason, viz: that the eye or the mind, istine- 
* For the preceding articles on this subject, see II, xlvii, 68, 153, and HI, 1, 33. 
Am. Jour. Sc1.—Turep Sertes, Vou. II, No. 7.—Ju tyr, 1871. 
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