O. N. Rood on Dove’s Theory of Lustre. 341 
them here. 
S we are accustomed to see gold tinted variously from nearly 
a yellow as in gold-leaf, to almost a copper-hue as in some speci- 
mens 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 homogenious 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 pre- 
pared paper, acquire the lustre and appearance of yellow plates 
in the shade, and reflecting images of dark objects. 
@ 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 sur- 
prising perfection the appearance of a glazed plate of chocolate. 
he 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 ;°, inch broad and one inch long, their 
distance apart haiti 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 tack scale. a 
- A somewhat inferior imitation of antimony was given b 
No. 1 blue and black scale, with gray Nos. 18 to 20, or by using 
No. 17 blue and black scale with white. 
* P.281, Physiologische Optik (Eneyklopddie der Physik, Leipzig, 1860.) 
