CHAP, v.] TEE STEREOSCOPE. 283 



from the same point of view. This is an essential condition of stereo- 

 scopic vision ; if it be realized, the superposition of the two images will 

 occur as in nature. We shall have before us, not a flat representation, 

 but a vision in relief, more life-like and vivid, in proportion as the 

 reproduction of the pictures with their details of light and shade is 

 faithful. IT they are not coloured one may fancy one sees objects in 

 marble, a sculptured reproduction of nature. 



Wheatstone's reflecting stereoscope was very soon modified, or at 

 least the principle on which it was constructed has been the basis of 

 a move handy and more perfect instrument, the invention of Sir 

 David Brewster, and this was still further perfected by two 

 French opticians, Soleil and Dubosq. 



But before describing the refracting stereoscope, a simple process 

 which enables us to realise the stereoscopic vision of images may be 

 referred to. We require, for this, to place two drawings side by side, 

 as is done in Fig. 210, and to interpose a diaphragm, a bit of paper 

 or card-board, on 'the middle line between the two eyes. After some 

 seconds the two images are superposed, and stand out in relief. Still 

 it is a fatiguing exercise for the eyes, and stereoscopes, as now con- 

 structed, have a marked advantage over this elementary stereoscopic 

 process. 



II. BKEWSTER'S REFRACTING STEREOSCOPE HKLMIIOLTZ'S 

 STEREOSCOPE PSEUDOSCOPE. 



We now arrive at Brewster's stereoscope. Here, Fig. 211, the 

 two images are not examined by reflection on two mirrors, but 

 directly, by placing the eyes before two lenses, two portions A A' 

 of a prism or a converging lens. From similar points, C C', on 

 each stereoscopic view a ray of light proceeds, which is refracted 

 by each prism, and gives rise to an image in the eye which is 

 formed at the same point, beyond the plane of the drawing, at C. 

 The same thing happens with all the corresponding parts of the 

 picture, so that the two stereoscopic views are depicted simultaneously 

 at a, b, the right view on the right retina, the left on the left retina. 

 Perfect vision in relief is the result, especially if the pictures are 



