J. LeConte on Binocular Vision. 321 
smaller, until when the point of sight is at the screen, fig. 4, 
then the images will completely unite, and the screen become en- 
lirely opaque. This is shown in the visual result, fig. 5. 
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If next we use fwo fingers, one of each hand, and gaze again at 
the wall, we will see four images all transparent. Now approx1- 
mate or separate the two fingers until the two middle images 
unite; we will have three images, the middle one opaque, the 
other two transparent. The reason is obvious. he middle 
one is opaque because a portion of the wall is concealed by it 
from both eyes. This portion of the wall is concealed from the 
he principles (1, 2, 3) laid down in the early part of this 
paper, together with the explanation of transparent double 
images just given, furnish, we believe, the key to all M. Pictet's 
experiments. We will make the application only to those 
which he thinks most conclusive of the existence of illusive 
images. We will first give his experiments and his conclusions 
as fairly as we can, and then will proceed to give our Own expla- 
nation. The following experiments M. Pictet thinks eee : 
ace an opaque screen S (fig. 6) agaist the nose n 1n a 
median plane of sight, in such wise that the object A may be 
