RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 59 
ously, and (3 more so than a ; its scale of transcalescence is therefore in analogy with 
its colour. It has been already mentioned that a pale yellow glass insulated a spot 
coincident with the fiducial point Y. A solution of muriate of chromium transmitted 
a and (3, the former most copiously, but stopped the rest. 
14/. In the mode of observation above described the thermic spectrum is rendered 
only transiently visible ; ample time, indeed, is afforded for all needful measurements, 
and for any examination, however minute ; but it seemed desirable to obtain a per- 
manent impression or a fixed thermograph, capable of being preserved as a record. 
In this I have been to a certain extent successful, and I doubt not shall very speedily 
be enabled to accomplish completely this desirable object. The method which I have 
hitherto found to succeed best, is to dissolve in the alcohol used for washing the paper 
a small portion of some colouring matter, which being deposited in the pores of the 
paper at those points where the evaporation goes on most rapidly, in greater quantity 
than where it proceeds slowly, becomes accumulated on those points by successive 
washes to such an amount as to indicate by a marked difference of colour the distri- 
bution of the calorific rays. The colouring matter of the Viola tricolor is very well 
adapted for this purpose, as it does not develop its colour from the alcoholic tincture 
immediately, but requires time for oxidation, so that the experiment is not interfered 
with by the paper being discoloured while exposed to the spectrum. 
148. To exhibit the polarization of solar heat, the following method was employed. 
In place of the prism, a plate of glass inclined at the polarizing angle to the axis of 
the lens was substituted, and intermediate between the lens and its focus a glass 
plate, also inclined to its axis at that angle, but in a plane at right angles to that of 
the first reflexion, was adapted. The focus, after reflexion at this plate, was received 
on prepared paper, when it formed a faint image, which of course would have been 
invisible were all the rays completely polarized. As it was, it proved too feeble to act 
thermographically on the paper when wetted with alcohol, thereby showing that no 
appreciable quantity of heat had undergone reflexion ; in other words, that the thermic 
as well as luminous rays had received the polarized character. A plate of mica was 
now interposed between the lens and the first glass-plate, so inclined as to polarize 
at an angle of 45° a considerable portion of the reflected beam, and thereby render it 
susceptible of reflexion at the second glass-plate. And it was now found that the 
focal image, which in this arrangement was considerably bright, had acquired the 
power of imprinting itself thermographically on the paper, and leaving there a well- 
defined circular white spot. The same effect was obtained when an unannealed glass 
disc was used to disturb the polarity of the cone of rays. 
J. F. W. H. 
