344 O. N. Rood on Dove’s Theory of Lustre. 
- 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 
for. 
: 5. If asheet 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. Ifaroll of black paper like the above, but coarser in its 
. markings, be brightly illuminated on oneside and viewed through 
deeply colored plates of glass (red, green, blue,) in a few seconds 
it appears lustrous resembling a roll of polished zine which has 
been irregularly and deeply corroded by an acid. Upon remov- 
ing the giass the surface of the paper appears lustrous 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. us Helmholtz remarks* upon the com- 
bination of two colored surfaces in monocular vision by m 
of a simple instrument he figures:—‘ It is particularly favorable 
when the drawings, or spots on the two surfaces are made to 
shift their position. - Then we often believe that we see both 
colors simultaneously in the same place, the one through the 
other. We have an impression in such cases of seeing objects 
through a colored vail or reflected from a colored surface.” 
I found in fact that by placing stereographs consisting of col- 
ored paper for one eye and a photographie drawing of tin-foil 
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 B. Differently colored papers are placed 
at K and Y; one is seen through the plate, 
and the other by reflexion from it. The im- 
ages are made to overlap and their intensity x 
is regulated by altering their distance from F, 
* Physiologische Optik, p. 278. 
