162 ANNUAL OF SCIENTIFIC DISCOVERY. 



the axes of both forming one and the same straight line. But it is 

 altogether impossible to obtain a relief in this case. A hexagon 

 embracing a six-pointed star is always obtained, the sides of which all 

 consist of red and blue lines running alongside each other in contact. 

 When this complicated figure was viewed through a blue glass, the 

 convex relief, bounded by red lines, started forth ; when a red glass 

 was used, the hollow pyramid with blue edges was observed. In the 

 first case the blue lines vanished almost completely in the blue light ; 

 while the red, whose rays were intercepted by the glass, acted as black, 

 and became subjectively colored red. In the second case the red 

 lines nearly disappeared in the red light ; and the blue, subjectiveJy 

 colored, combined themselves to a relief. 



To understand what has here been said regarding subjective color- 

 ing, attention to the following facts is necessary. If the diffused 

 daylight be completely shut out from the eyes, and a drawing in black 

 outline on white ground be viewed through a colored glass, the relief 

 is seen with black edges ; but when the glass is held at some distance 

 from the eyes, so that the diffused daylight shall also reach them, the 

 black hues assume a vivid subjective coloring, which becomes stronger 

 the longer the drawing is regarded. When the glass is colored blue 

 by cobalt, the lines appear red ; when the glass is a ruby-red, the out- 

 line appears bluish-green. 



The result of the above experiment with the blue and red pyramids 

 is remarkable. Each eye has two drawings presented to it, and a 

 double combination is thus possible. When the identity of outline is 

 preserved by the eye, and no regard is paid to the difference of tint, 

 two plane figures composed of different colors must be observed. 

 This is the case when the intensity of both colors is nearly the same. 

 When, however, the intensity is very different, such, for example, as 

 that brought about by the red and blue glasses in the case under 

 consideration, the identity of the outline is overcome by the tendency 

 to form a relief. 



The projection for one eye was drawn in white lines upon a black 

 ground, and for the other eye with black lines upon a white ground. 

 A most remarkable result was obtained by the stereoscopic combination 

 of both. The relief started into existence with surfaces which shone 

 like graphite, having their edges formed of dazzling white and deep 

 black lines which run parallel and in contact with each other through- 

 out. When the black leaf with the white lines is placed before the 

 left eye, and the white leaf with the black lines before the right eye, 

 the white lines in the relief lie to the right of the black ones. When 

 the leaves are changed, the relative position of the black and white 

 hues is also changed ; hence the lines appear always pushed aside 

 cross-wise. Exactly as in the case of black and white, combinations 

 of both of these with other colors are obtained, and combinations 

 of the latter with each other. To obtain the combination of dioptric 

 colors with white and with each other, drawings in white outline 

 on a black ground are made use of. When white is to be combined 

 with another color, a glass of the required tint is placed before 





