434 COMBINATION OF RETINAL PICTURES :—STEREOSCOPE. 
passed away, the conformity of the two eyes be restored (as 
by the operation for the care of squinting), there is double 
vision for some little time, although the two parts of the 
retinae, which originally acted together, are now brought to 
do so again. 
560. That the combination of the two images must be 
effected by an operation of the mind, is evident from another 
circumstance. It is easy to show that no near object is seen 
by the two eyes in exactly the same manner. Thus, let the 
reader hold up a thin book, in such a manner that its back 
shall be exactly in front of his nose, and at a moderate 
distance from it; he will observe, by closing first one eye and 
then the other, that his view of it is very different, according 
to the eye with which he sees it. With the right eye he will 
see its back and right side, the latter very much foreshortened, 
but none of the left side; whilst with the left eye he will see 
its back and left side, the latter also foreshortened, but none 
of the right side. Hence if he were to draw a perspective 
view of the object as seen by each eye, the two delineations 
would be very different. But on looking at the object with 
the two eyes conjointly, there is no confusion between these 
pictures, nor does the mind dwell upon either of them singly ; 
the union of the two gives us the idea of a solid projecting 
body—such an idea as we could have only acquired otherwise 
by the exercise of the sense of touch. 
561. That this is really the case, has been proved by ex¬ 
periments with the very ingenious instrument (invented by 
Professor Wheatstone) known as the Stereoscope. In its 
original form this consisted of two plane mirrors, inclined with 
their backs to one another at an angle of 90°, the point of 
meeting being opposite to the middle of the forehead. Two 
drawings representing the different perspective views of any 
solid object, as seen by the two eyes, being placed before 
these mirrors, in such a manner that their images are re¬ 
flected to the two eyes respectively, and are made to fall 
on corresponding parts of the two retinas as the two images 
formed by the solid object itself would have done, so 
that their apparent places are the same,—the mind perceives 
not one or other of the single representations of the object, 
nor a confused union of the two, but a body projecting in 
relief,\ the exact counterpart of that from which the drawings 
