342 O. N. Rood on Dove's Theory of Lustre. 
8. Tarnished zinc surfaces may be imitated by the use of grey 
No. 5 with No. 18 blue and black seale. 
4. Ultramarine paper with some of the lighter violet blues 
gave an imitation of blue glass. The idea of blue polished 
glass was also obtained by using in combination with the ultra- 
marine paper No. 1 of the yellow and black scale. 
will mention here that the stereoscopic union of this blue 
with yellow paper, never induced in my mind the idea of green. 
I made some experiments to ascertain how far the stereoscopic 
mixture of two masses of different colored light corresponded to 
their true mixture by the method of rapid rotation, use being 
made of the imitations above described. It is however so diffi- 
cult to compare a varying with a fixed tint that I will not record 
the results obtained; in many cases a certain moderate amoun 
of agreement in the resultant tints was observed. Briicke found 
that when a deeply colored yellow glass was held before one eye, 
a blue cobalt glass before the other, that a landscape viewed 
through this combination was simply darkened in appearance. 
I repeated this experiment with similar glasses and obtained a 
like result; objects appeared darkened, but in their natural 
_ colors, though sometimes the blue or yellow tint predominated a 
little. But when I presented to a single eye these two masses of 
light a very different result was obtained; the plates of glass 
were attached to a blackened disc opposite suitable perforations, 
and it was set in rapid rotation; a landscape viewed through it 
appeared deep purple, though not a trace of this color was to be 
perceived in the binocular use of these glasses. 
hen these two glasses were held before the same eye, a 
landscape viewed through them was very much darkened but 
scarcely colored. 
Sir Davin Brewsrer’s Tuzory or Lustre. 
. en heh sh oa of Sir David’s experiment, Dove 
as shown that the objection founded on it is without weight— 
(p. 8, Optical Stndien). gia Ween 
_In repeating Brewster’s experiment I always obtain the oppo- 
sue result ; in combining uniform black and white s , with- 
ut drawings, I always obtain a distinct impression of lustre, 
_ like that of the blackened mirror of a polariscope, and in strict 
ordance with Dove’s theory, when the black field is so dark- 
ened that no light is sent from it to the eye, this lustre vanishes, 
