96 W. B. Rogers on Binocular Vision. q 
the best practicable adjustment of the lenses and diagrams can 
only secure the direct optical coincidence of the virtual foci first 
referred to for certain, points or lines of the drawings, and must 
leave all the rest to be combined by the method of constrained a 
unusual adjustments of which I have been speaking. 
The same thing « occurs in using the reflecting : ero 
Wheatstone (fig. 7), if after adjusting the diagrams so as to — 
cause the right and left i images to appear side by side instead of 
being coincident, we then by constrained vision bring them to 
gether so as to form the resultant in a position nearer or more Te — 
mote than the plane in which they are situated. Thus if mand — 
“n denote the virtual foci or images of P and Q, formed by reflex 
ion from the inclined mirror 4. : 
C and D, these by ordinary Y 
i pres to the eyes 
at A an as separate ob- 
jects actually at ™ 
id n. But readily x . fas oe 
unite them beyond-R b 
suming the line VR to bi- 
sect the angle of the mirrors 
and Hi] to be parallel , A oo 
to VR and equidistant from , 
it, and supposing PQ, to be at right angles to it, it is evident a 
P an , considered as points in the drawings, will have their vite 
tual foci ‘optically coincident at r, and will be seen as oue in 
position. If however we move them backwards on. the ee : 
from PQ, their images will be found at m and n, and so 
moved forwards to P»Qz2, the same distance in front of PQ, 
images will take the positions » and m, the reverse of the tel 
When however we adjust them in the position P2 and Q:, the 
BE 
ahd images will be seen optically a at ca and so if 
at P iss they will form a united image ai 
Bios these relations we draw the sues conse that sur 
posing the stereoscopic drawings to be duly adjusted in ‘the slides : 
the points situated at P and Q and such other pairs ints 
are equidistant from PQ in opposite directions will all be uo 
pair by pair in different positions on the line HK; all Bnet 
points situated in front of PQ or behind it, or on 
