292 RECE]^ 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. Stereoscopic draw- 

 ings 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 direc- 

 tions, 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 eve may receive this regularly reflected light 

 and the other nut ; 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 poli^^hed 

 surface. 



Now if there were a complete combination of the 

 impressions produced upon both retinae, the union of 



