W. B. Rogers on Binocular Vision. 91 
% 
As the lateral images tend to withdraw the eyes from the com- 
pound one situated between them, and thus to distract the ob- 
server’s attention, it is necessary, for the most satisfactory results, 
to exclude these from the view. In the simple form of stereo- 
scope about to be described, this is effected in such a manner that 
by suitable adjustments we may obtain the resultant image either 
at a greater or less distance from the eyes than that of the draw- 
ings from which it is formed. It will be seen that in general plan 
this apparatus bears some resemblance to the phantascope of Prof. 
Lock atter however is only adapted to the formation of 
acompound image between the drawings and the eyes, and it — 
does not exclude the lateral pictures. ha 
3. Description of the sliding stage Stereoscope. ll 
This instrument is represented in fig. 3 where AB 
strip of light hard wood about 3 ft. long and from 23 to 3 gy -4 
d D of the 
broad to which are attached two square pieces C t 
same material each about 6 inches square, fittin aidutly upon the 
strip by slits at B and 7 so as to be ajunibieat vain distances 
from A. InC is cut a rectangular opening m about 14 inches 
wide and 2 inches long. A narrow slip is fixed on the stage D 
at o and another on C at p for the prirpose rting the dia- 
grams when the instr ts held in a slightly inclined position 
with the end B a little downwards. 
. a iG ay 
re) 
When the diagrams are to be combined at a point farther from 
_ the eyes than their actual distance they must be placed at ” on 
the stage C and the instrument turned around so as to bring 
above m. Should the distance between their corresponding 
points be much less than the interval separating the two eyes, the 
union will be readily effected by bringing A close to the face, 
midway between the eyes, and looking past the diagrams towards 
the moveable pin S or to some point between it and m, which in 
the rapid adjustments of the axes is almost instantly found. In 
this use of the instrument the part Ar of the central strip con- 
ceals both the lateral images, the right eye being permitted to 
see only the right hand figure and the left eye that on the left 
hand, and these by a due convergence of the axes are united ina 
single stereoscopic resultant. When the component diagrams are 
farther apart or when the stage itself is moved to a greater dis- 
tance from A, i 
the point of convergence proper to their union is 
