404 Prof. Emerson's Description of his 
We propose in the present article—Ist, to account for these 
imperfections in the stereoscope, and 2d, to describe a simple 
modification of the instrument, which will remove them com- 
pl 
~e 
a 
one measured on a perpendicular line, it is much more serious 
the eyes are not accustomed to move in this direction. independ- 
ently of each other; so that if the imperfect mounting of the 
lenses causes a variation on this line of from one to two tenths 
of an inch it will be almost impossible for the observer to unite 
the views. An easy method of testing an instrument for t 
fault is to draw upon a piece of white paper the size of a stereo 
scopic slide, two series of short lines, each series being 2°6 inches 
from the other, and the lines in each series being drawn at cof 
responding distances from each other, thus; 
Upon placing this in the stereoscope, and looking at it, if eo 
lines unite instantly with no variation, it proves the an 90 : 
this fault; but if, on the contrary, we obtain as a resultan 
view of more than four lines, thus: 
we may be certain that the lenses are not properly mounted, and 
will give in usage little or no satisfaction. The instramer 
however, may be perfect as to its lenses, and a difficulty ea 
xperien in pi r 
