550 OF SENSATION. 



operation for the cure of squinting), there is double vision for some little 

 time, although the two parts of the retinae, which originally acted toge- 

 ther, are now brought into their pristine position. 



964. But the images thus combined are far from being identical ; and 

 one of the most remarkable of all our intuitive perceptions, is that by 

 which they are reconciled and combined, and are caused to give rise to 

 an idea that differs essentially from either image. No near object can 

 be seen by the two eyes in the same manner ; of this the reader may 

 easily convince himself, by holding up a thin book, in such a position 

 that its back shall be in a line with the nose, and at a moderate distance 

 from it ; and by looking at the book, first with one eye, and then with 

 the other. He will find that he gains a different view of the object with 

 each eye, when used separately ; so that if he were to represent it, as 

 he actually sees it under these circumstances, he would have two per- 

 spective delineations differing from one another, because drawn from 

 different points. But on looking at the object with the two eyes con- 

 jointly, there is no confusion between these pictures ; nor does the mind 

 dwell upon either of them singly ; but the union of the two intuitively 

 gives us the idea of a solid projecting body, such an idea as we could 

 only have otherwise acquired by the exercise of the sense of touch. 

 That this is really the case, has been proved by experiments with a very 

 ingenious instrument, the Stereoscope, invented by Prof. Wheatstone ; 

 which is so contrived, as to bring to the two eyes, by reflection from 

 mirrors, two different pictures, such as would be accurate representa- 

 tions of a solid object, as seen by the two eyes respectively. When the 

 arrangement is such, as to bring the images of these pictures to those 

 parts of the retinae, which would have been occupied by the images of 

 the solid (supposing that to have been before the eyes), the mind will 

 perceive, 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 were made. Thus 

 the combination of the two pictures, and the perception of an object 

 different from either of them, is effected by a mental process of an in- 

 stinctive kind, of the nature of which we know nothing further. 



965. When two pictures, representing dissimilar objects, are pro- 

 jected upon the retinae of the two eyes by means of the Stereoscope, the 

 result is a curious one. The mind perceives only one of them, the 

 other being completely excluded for a time ; but it commonly happens 

 that, after one has been seen for a short period, the other begins to 

 attract attention and takes its place, the first entirely disappearing ; so 

 that there is no confusion or intermingling of images, except at the 

 moment of change. The Will may determine, to a certain extent, 

 which object shall be seen ; but not entirely ; for if one picture be more 

 illuminated than the other, it will be seen during a larger proportion of 

 the time. An interesting variation of this experiment may be made, 

 without the aid of the Stereoscope, by holding a piece of blue glass 

 before one eye, and a piece of yellow glass before the other. The re- 

 sult will be, not that everything will be seen of a green colour, but that 

 the surrounding- objects will be seen alternately blue and yellow; or 

 sometimes the field of vision will be blue spotted with yellow, alterna- 



