44 On Dove's Theory of Lustre. 



and be steadily regarded for some minutes by one eye, it assumes 

 a red-orange hue, and appears suspended over the blue paper 

 and nearer to the eye than the latter ; at the same instant it 

 appears lustrous like crumpled mica. The illusion with me 

 often lasts half a minute in great perfection ; this is particularly 

 the case when the eye is not quite accurately focused on the 

 paper. 



3. If a sheet of this prepared paper be brightly illuminated 

 by light from a window, and be held so near one eye as to pro- 

 duce indistinct vision, it often apparently becomes highly lus- 

 trous. In this case enlarged images of the white and grey 

 points are formed on the retina, which overlap, so that again we 

 have two masses of light, one passing through the other. 



4. If a roll of black paper like the above, but coarser in its 

 markings, be brightly illuminated on one side and viewed through 

 deeply coloured plates of glass (red, green, blue), in a few 

 seconds it appears lustrous, resembling a roll of polished zinc 

 which has been irregularly and deeply corroded by an acid. 

 Upon removing the glass, the surface of the paper appears lus- 

 trous for an instant. 



5. A sheet of the finer variety of this prepared paper viewed 

 through a large rhomb of calc-spar, gives often in spots the ap- 

 pearance of lustre, particularly when the head of the observer, 

 or the rhomb, is slightly moved. Some persons compared this 

 to the appearance of water. 



It would seem probable that in all cases where two masses of 

 light reach a single eye, one passing through the other, particu- 

 larly when there is any perception of their individuality, that the 

 appearance of more or less lustre is produced, though from habit 

 we often overlook it. Thus Helmholtz remarks* (upon the com- 

 bination of two coloured surfaces in monocular vision by means 

 of a simple instrument he figures), " It is particularly favourable 

 when the drawings, or spots on the two surfaces, are made to 

 shift their position. Then Ave often believe that we sec both 

 colours simultaneously in the same place, the one through the 

 other. We have an impression in such cases of seeing objects 

 through a coloured veil or reflected from a coloured surface.-" 



I found, in fact, that by placing stereographs consisting of co- 

 loured paper for one eye and a photographic drawing of tinfoil 

 for the other in this instrument, that lustre could be perceived, 

 particularly with the imitations of copper. 



The diagram represents the instrument referred to ; it consists 



of a plate of glass, P, with parallel sides, which is properly 



supported over a blackened board 13. Differently coloured papers 



are placed at K and Y ; one is seen through the plate, and the 



* Physiologische Optik, p. 2/3. 



