42 Prof. Rood on some Experiments connected 



3. Tarnished zinc surfaces may be imitated by the use of grey 

 No. 5 with No. 18 blue and black scale. 



4. Ultramarine paper, with some of the lighter violet-bines, 

 gave an imitation of blue glass. The idea of blue polished glass 

 was also obtained by using in combination with the ultramarine 

 paper No. 1 of the yellow and black scale. 



I 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 coloured 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 amount 

 of agreement in the resultant tints was observed. Biiicke found, 

 when a deeply-coloured yellow glass was held before one eye 

 and 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 

 colours, 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 disk opposite suitable perforations, 

 and it was set in rapid rotation ; a landscape viewed through it 

 appeared deep purple, though not a trace of this colour was to 

 be perceived in the binocular use of these glasses. 



When these two glasses were held before the same eye, a 

 landscape viewed through them was very much darkened but 

 scarcely coloured. 



Sir David Brewster's Theory of Lustre. 



Sir David Brewster opposes Dove's theory of lustre, as he has 

 found that when black and white surfaces without drawings are 

 combined in the stereoscope, no lustre is produced. The lustre, 

 then, according to this philosopher, is due not to one mass of 

 light passing through another, but to the effort of the eyes to 

 combine the stereoscopic pictures. 



Admitting the correctness of Sir David's experiment, Dove 

 has shown that the objection founded on it is without weight 

 (p. 3, Optical Studies). 



In repeating Brewster's experiment I always obtain the oppo- 

 site result; in combining uniform black and white surfaces, with- 

 out drawings, I always obtain a distinct impression of lustre, 

 like that of the blackened mirror of a polariscope, and in strict 

 accordance with Dove's theory ; when the black field is so dark- 



