Prof. Dove on the Stereoscopic Combination of Colours. 243 



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

 combinations of both of these with other colours are obtained, 

 and combinations of the latter with each other. To obtain the 

 combination of dioptric colours 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 colour, a glass of 

 the required tint is placed before one eye, while the drawing is 

 viewed by the other eye naked. When different colours are to 

 be combined, suitable glasses are placed before both eyes. The 

 most beautiful result is obtained when the colours produced 

 by a deep blue and a red glass are combined ; the relief stands 

 forth illuminated with violet light and with splendid edges of 

 red and blue, which run alongside each other in contact. In 

 the case of colours which nearly approach each other, the edges 

 are also formed by those double and differently coloured lines. 

 One result is always observed, — the lines appear pushed aside 

 cross-wise ; that is, the colour obsei-ved by the left -eye appears 

 to the right, and that observed by the right eye appears to the 

 left. 



The following remarkable fact has been observed by M. Dove, 

 and his observation has been corroborated by others. The pro- 

 jections of a convex and concave pyramid for the right eye were 

 drawn upon the same base, and on a second leaf the projection 

 of a convex only for the left eye. In the stereoscope, thei-efore, 

 a convex pyramid was seen, and on the base of the same the 

 projection of a concave one. When the ruby-red glass was brought 

 before the left eye, while the former drawing was regarded by the 

 naked right eye, both the pyramid and the projection were ob- 

 served ; but it depended entirely on an act of volition whether 

 the pyramid was observed with red and white boundaries and 

 the projection in white lines, or the pyramid with white bound- 

 aries and the projection in red and white outline. It hence 

 appears that a projection as contour can combine itself with 

 another as colour to form a relief. 



The same phsenomena which we have observed with objective 

 colours exhibit themselves with subjective colours also. On 

 viewing the drawings formed in black outline on white gromid 

 through the ruby-red glass with one eye, and through the glass 

 coloured blue by cobalt with the other, ])ermitting the diffused 

 daylight at the same time to strike the eyes, the relief is observed 

 with coloured double parallel lines as edges, as in the other in- 

 stances : the crossed position of the lines is also observed here ; 

 HO that when the red glass is held before the left eye, and the 

 . blue glass before the right, the bluish-green lines appear to the 

 right of the red ; — it will be remembered that the subjective tint 



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