434 COMBINATION OP RETINAL PICTURES : STEREOSCOPE. 



passed away, the conformity of the two eyes be restored (as 

 by the operation for the cure 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 retinae 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 



