320 J. LeConte on Binocular Vision. 
seen by each eye, either by true or by illusive images; we say, 
every part of the wall is seen, not by each eye, but by the &- 
nocular observer ; not some parts by true and some by illusive 
images, but only by true images. ~ ; 
If instead of a finger we use a screen several inches wide 
(wider than the interocular space), then the double images wil 
not entirely separate. They will slide over each other hete- 
ronymously through a space equal to the interocular space (2). 
The overlapping area will be opaque because it covers a portion 
of the wall concealed from both eyes; the rest will be transpar- 
ent. ‘The visual result is repre- 
$ s’ , sented by fig. 1, in which SS 1s 
the right-eye image of the screen, 
S’S’ the left-eye image, and 8’S 
the overlapping area. These 
facts are more completely repre- 
sented by my method in figs. 2 
and 3, of which fig. 2 represents 
the actual relation of parts, and fig. 3 the visual result. In fig. 
, Hand L are the right and left eye respectively, n the nose, 
‘ 
ae) 
S Ss! 
m the median line, »v the visual lines, SS the screen. Fig. 3 
will readily explain itself if the reader will call to mind that in 
| : figures representing visual results capitals represent 
combined images, small italics right-eye images, and dashed 
italies left-eye images, If the optic axes be gradually 
converged, as already explained (3), these heteronymous imag® 
will slide over each other homonymously, making the opaque 
area larger and larger, and the transparent margins smaller and 
=} 
