436 PROFESSOR LUDWIG MOSER ON VISION, 
cale-spar, which is depicted in a very beautiful manner. I gene- 
rally made this experiment by fastening, just before the lens of 
the camera obscura, the common apparatus, consisting of cale- 
spar, with two parallel faces, standing perpendicular to the axis 
between two Nicols rhombs, and directing the whole towards a 
clear sky. Moreover, I succeeded in representing the figures of 
rapidly cooled glasses, sometimes in such a manner, by placing 
a prism of calc-spar between the glass and the lens, as to obtain 
both complementary figures at once. These experiments were 
completely successful, and prove very evidently that if there are 
chemical rays they are polarized in precisely the same manner 
as the luminous ones. For instance, the black cross in the image, 
produced by a quickly-cooled cube, shows that the chemical rays 
are just as little reflected from the cross mirror of the polarizing 
apparatus as those that are luminous. 
In repeating these experiments it may perhaps be desirable to 
know the time necessary for their performance, so that there 
may be some guide by which the operator may be directed. The 
lens of my camera obscura had a focal distance of 3-90 inch. 
and an aperture of 0°501 inch. By means of a prism of calc-spar, 
when chloride of iodine was used, and the object was a white 
bust placed in the sun, the double image was obtained within a 
minute. The annular system was depicted on iodide of silver 
without the assistance of chloride of iodine, and on a dull day 
in the space of two hours and a half,—the cooled glasses in about 
the same time. We see, therefore, that sometimes hours are 
necessary for these experiments, if the intensity of the light is 
very small, and chloride of iodine has not been employed. It is 
therefore necessary tu perform the experiments in a darkened 
room, and to take care to exclude all lateral light. 
As we may conclude from the foregoing experiments that the 
chemical rays do not exhibit any difference from the luminous 
rays, as far as regards reflexion, refraction, interference, and 
polarization, we cannot possibly allow the assumption of their 
being a peculiar class of rays. The matter would stand in a quite 
different light if we were to adopt the view which I have given 
of the action of the rays on the retina. It has been often stated 
that we must consider a luminous body as one from which are 
emitted rays of the most various kinds, possessed of the most 
different rapidities of translation and oscillation. The fact that 
the light of those stars towards which the earth is moving, pos- 
