256 RECENT PROGRESS OF THE THEORY OF VISION. 



that they seem to be at rest. Hence there cannot be the 

 slightest movement of the eye, while the spark lasts, which 

 can possibly be recognised ; and yet we receive the complete 

 impression of stereoscopic relief. 



Secondly, such a combination of the sensations of the two 

 eyes as the anatomical hypothesis assumes, is proved not to 

 exist by the phenomenon of stereoscopic lustre, which was also 

 discovered by Dove. If the same surface is made white in one 

 stereoscopic picture and black in another, the combined image 

 appears to shine, though the paper itself is quite dull. Stereo- 

 scopic drawings of crystals are made so that one shows white 

 lines on a black ground, and the other black lines on a white 

 ground. When looked at through a stereoscope they give the 

 impression of a solid crystal of shining graphite. By the same 

 means it is possible to produce in stereoscopic photographs the 

 still more beautiful effect of the sheen of water or of leaves. 



The explanation of this curious phenomenon is as follows : 

 A dull surface, like unglazed white paper, reflects the light 

 which falls on it equally in all directions, and, therefore, always 

 looks equally bright, from whatever point it is seen ; hence, of 

 course, it appears equally bright to both eyes. On the other 

 hand, a polished surface, beside the reflected light which it 

 scatters equally in all directions, throws back other beams by 

 regular reflection, which only pass in definite directions. Now 

 one eye may receive this regularly reflected light and the other 

 not; the surface will then appear much brighter to the one than 

 to the other, and, as this can only happen with shining bodies, 

 the effect of the black and white stereoscopic pictures appears 

 like that of a polished surface. 



Now if there were a complete combination of the impressions 

 produced upon both retinae, the union of white and black would 

 give grey. The fact, therefore, that when they are actually 

 combined in the stereoscope they produce the effect of lustre 

 that is to say, an effect which cannot be produced by any kind 

 of uniform grey surface proves that the impressions on the two 

 retina? are not combined into one sensation. 



That, again, this effect of stereoscopic lustre does not depend 



