with Dove's Theory of Lustre. 41 



On the other hand, by diminishing the brightness of the 

 yellow paper, the black tint remaining constant, the idea of a 

 polished golden plate in the. shade, or so placed as to reflect the 

 image of some dark object, is produced. Thus we may descend 

 through the circles of Chevreul to the 7th, when by combining 

 the orange-yellow of that circle with No. 21 of the grey scale, 

 the idea of a golden plate much shaded is produced. I con- 

 structed tables expressing the effects produced by varying the 

 intensity of the two components; but it is not worth while to 

 introduce them here. 



As we are accustomed to see gold tinted variously from nearly 

 a yellow as in gold-leaf, to almost a copper hue as in some spe- 

 cimens of our American coin, so the tint of the paper placed in 

 the stereoscope may be varied within certain limits, without 

 greatly affecting the results. 



Prof. Helmholtz, in his admirable work on Physiological 

 Optics*, mentions that by a peculiar arrangement he was able to 

 cause the homogeneous golden-yellow light of the spectrum to 

 appear brown, proving thus that the tint brown is only weak 

 yellow light. These stereoscopic experiments give us, on the 

 other hand, the means of apparently converting brown into a 

 metallic golden yellow; for many specimens of even brown 

 wrapping paper, when combined in the stereoscope with very 

 black prepared paper, acquire the lustre and appearance of yellow 

 plates in the shade, and reflecting images of dark objects. 



In the same manner, and corresponding to the investigations 

 of Helmholtz, I found that the stereoscopic union of black 

 glazed paper with red (No. 14, red and black scale), imitated 

 with surprising perfection the appearance of a glazed plate of 

 chocolate. 



The chromatic scales of Chevreul furnish us with a ready 

 means of combining in rapid succession in the stereoscope a 

 great number of definite tints; thus by cutting in a card-board 

 two parallel apertures fV inch broad and 1 inch long, their 

 distance apart being 2"6 inches, and pasting under one of them 

 black prepared paper, the other can be brought over any de- 

 sired tint and the effect noted. 



1. In this way I found that a pretty good representation of 

 the appearance of slightly tarnished lead was produced by the 

 stereoscopic union of grey No. 18 and No. 4 on the blue-violet 

 and black scale. 



2. A somewhat inferior imitation of antimony was given by 

 No. 1 blue and black scale, with grey Nos. 18 to 20, or by using 

 No. 17 blue and black scale with white. 



* P. 281. Thysiologische Optik(Encijklopddic der Physik. Leipzig, 1860). 



