THE STEREOSCOPE. 



photographic instruments be then placed one on each of these 

 lines, with their optic axes in the directions of the lines respec- 

 tively, and therefore converging towards the same point of the 

 object, and let the distances of their object glasses from that point 

 be equal. The optical pictures which they will produce will in 

 that case be those which would be seen by two eyes, right and 

 left, having a distance apart equal to the distance between the 

 object glasses of the two photographic instruments. 



When the pictures are thus produced on a small scale they are 

 placed in the stereoscope, the eye-glasses of which will have the 

 effect of causing them to be viewed in lines converging at the same 

 angle, as that formed by the optic axes of the two photographic 

 instruments by which the pictures were produced. 



11. It will be manifest, then, that the impression produced by 

 the view of such pictures in the stereoscope will be such, as could 

 never be produced by the immediate view of the objects them- 

 selves, inasmuch as they could never be seen with any such 

 degree of binocular parallax, as that which has been given to 

 them by the relative position of the two photographic instruments. 

 This parallax will be greater than the natural binocular parallax 

 of the object, in the same proportion as the distance between the 

 centres of the object glasses of the two photographic instruments, 

 is greater than the distance between the eyes. Thus if, in taking 

 such a pair of stereoscopic views of a building, the distance 

 between the photographic instruments is 50 inches, the parallax 

 thus produced will be greater than the natural binocular parallax 

 in the proportion of 50 to 2, or 20 to 1, and so far as the percep- 

 tion of perspective and relief depends on binocular parallax, that 

 which is produced from viewing the pictures of the building in 

 the stereoscope, will be 20 times more strong and vivid than that 

 which is produced by the view of the building itself, seen from the 

 station at which the pictures are taken. 



It is then rigorously true, that the surprise and admiration 

 excited by the stereoscope, does not arise from the truth of the 

 picture which it presents, but from the strong exaggeration of 

 perspective and relief which it exhibits. It is very true that no 

 art of the draughtsman or painter could produce any such effects : 

 but it is equally true that no such effects could be produced by 

 the objects themselves. 



Among the most interesting and instructive as well as sur- 

 prising effects of the stereoscope, are those which it exhibits when 

 stereoscopic views of geometrical solid figures are exhibited in it. 

 The variety of these is endless. But since no mere verbal de- 

 scription could convey any adequate idea of them, we can only 

 invite the reader's attention to this class of objects. 

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