243 Dr. Tyndall on the Progress of the PMjsical Sciences : 



one and the same straight line. But it is altogether impossible 

 to obtain a relief in this case. A hexagon embracing a six- 

 poiuted star is always obtained, the sides of which all consist of 

 red and blue lines running alongside each other in contact. 

 "NATien 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 i*ays were intercepted by the 

 glass, acted as black, and became subjectively coloured red. In 

 the second case the red lines nearly disappeared in the red light ; 

 and the blue, subjectively coloured, combined themselves to a 

 relief. 



To understand what has been here said regarding subjective 

 colouring, attention to the following facts is necessary. If the 

 diffused daylight be completely shut out from the eyes, and a 

 dra\^^ng in black outline on white ground be viewed through a 

 coloured glass, the relief is seen with black edges ; but when 

 the glass is held at some distance from the eyes, so that the dif- 

 fused daylight sliall also reach them, the black hues assume a 

 vi\id subjective colouring, which becomes stronger the longer 

 the drawing is regarded. When the glass is coloured blue by 

 cobalt, the lines appear red; when the glass is a raby-red, the 

 outline appears bluish-green. 



The result of the above experiment with the blue and red 

 pj-ramids is remarkable. Each eye has two drawings presented 

 to it, and a double combination is thus possible. When the 

 identity of outline is presei-ved by the eye, and no regard is paid 

 to the difference of tint, two plane figures composed of diffei'ent 

 colours must be observed. This is the case when the iu tensity 

 of both colours is nearly the same. \^Tien, however, the intensity 

 is very different, such, for example, as that brought about by the 

 red and blue glasses in the case mider consideration, the identity 

 of the outline is overcome by the tendency to form a relief. 



The projection for one eye was di'awn 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 stereo- 

 scopic combination of both. The relief started into existence 

 with sui'faces which shone like graphite, having their edges 

 formed of dazzling white and deep black lines which run pa- 

 rallel and in contact with each other throughout. ^Alien 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. ^'\Tien 

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

 white hues i.s also changed ; hence the lines appear always pushed 



