NATURAL PHILOSOPHY. 16 







one eye, while the drawing is viewed by the other eye naked. When 

 different colors are to be combined, suitable glasses are placed before 

 both eyes. The most beautiful result is obtained when the colors 

 produced by a deep blue and a red glass are combined : the relief 

 stands fortli illuminated with violet light and with splendid edges 

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

 the case of colors which nearly approach each other, the edges are 

 also formed bv those double and differently colored lines. One result 



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is always observed the lines appear pushed aside cross-wise, that is, 

 the color observed 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 projections 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, therefore, 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 drawhi"- was regarded bv the naked rischt eve, both the 



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pyramid and the projection were observed ; but it depended entirely 

 on an act of volition whether the pyramid was observed with red 

 and white boundaries and the projection in red and white outline. It 

 hence appears that a projection as contour can combine itself with 

 another as color to form a relief. 



The same phenomena which we have observed with objective colors 

 exhibit themselves with subjective colors also. On viewing the draw- 

 ings formed in black outline on white ground through the ruby-red 

 glass with one eye, and through the glass colored by cobalt with the 

 other, permitting the diffused daylight at the same time to strike the 

 eyes, the relief is observed with colored double parallel lines as edges, 

 as in the other instances : the crossed position of the lines is also 

 observed here ; so 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 

 developed by the red glass is bluish-green, and by the blue glass, red. 



Why is it then, that the red and blue lines cannot be made to com- 

 bine, but always lie alongside each other crossed in the manner indi- 

 cated ? M. Dove finds the explanation in the non-achromatic nature 

 of the eye. That the eye is not achromatic has been known since 

 the time of Fraunhofer ; but a very simple way of proving the fact 

 was discovered independent by M. Dove and M. Plateau about 

 twelve years ago. If the flames of a candle be viewed through a 

 colored glass which permits the ends of the solar spectrum to pass 

 through it, but extinguishes the middle, at the distance of distinct 



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vision a violet flame is observed. At a greater distance a red flame 

 is observed within a larger blue one, which embraces the former on 

 all sides and becomes wider the further we recede from the flame. 

 Within the distance of distinct vision, on the contrary, the violet 

 flame is encompassed by a sharp red rim. From a medium distance a 



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