RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 53 
marked in some colours more than in others, and is particularly striking in the case of 
Indian ink. It depends obviously upon the water wherewith the colours are mixed, ren- 
dering, by its refractive power, the paper beneath more penetrable by the incident light, 
which entering its pores becomes absorbed and lost by numberless partial reflexions. 
131. If a paper so over-coloured be dried unequally, those parts which are dry first 
appear lighter than the rest ; and if a heated body be applied at any point behind the 
paper while wet, the increased evaporation at the points of contact will speedily effect 
the appearance of a white spot or figure, tracing out the form of the body, or the 
locus of incident heat where radiant from it ; and thus the effects of either conducted 
or radiant heat may be rendered sensible to the eye. It is of this property, traced a 
little further into its mode of operation, and exalted by certain processes, which I am 
about to describe, that advantage may be taken to afford a visible picture of the 
thermic spectrum. 
132. The paper best adapted of any which I have tried for placing in distinct evi- 
dence the calorific spectrum, is that called thin post, of the thinnest kind which will 
bear ink-writing, such as is sold for foreign correspondence. Its thickness, measured 
by the sphserometer, is 0*00136 inch. One side of this paper is to be blackened with 
Indian ink, or, which is better, smoked in the flame of oil of turpentine, or over a 
candle burning with a smoky flame, by drawing it often and quickly through the 
flame, giving it time to cool between each exposure, till it is coated on the under side 
with a film of deposited black, as nearly uniform as possible. A slip of the paper so 
prepared is to be stretched on a frame adjusted to the spectrum-apparatus described 
in the foregoing pages, so as to present its white side to the incident spectrum, keep- 
ing the blackened side hollow to admit air, and to avoid rubbing off the black coat*. 
A fiducial dot being made on it, and brought to coincidence with the standard yellow 
ray of the spectrum, a flat brush, equal in breadth to the paper, dipped in good recti- 
fied spirit of wine, is to be passed over the white surface till the paper is completely 
saturated, which will be indicated by its acquiring a uniform blackness in place of 
the white it at first exhibited. After a few moments’ exposure, a whitish spot begins 
to appear considerably below the extreme red end of the luminous spectrum, (sup- 
posing the violet end uppermost,) which increases rapidly in breadth until it equals 
the breadth of the luminous spectrum, and even somewhat surpasses it ; and in length, 
till it forms a long appendage exterior to the spectrum, and extends moreover within 
it up to and beyond the fiducial yellow. In this state, and just as the general drying 
of the paper begins, by whitening the whole surface, to confuse the appearances, a 
second, sudden, and copious wash of alcohol from above downwards must be applied 
without disturbing the spectrum, or in any way shaking the apparatus. The super- 
* The effect is more marked if the black side he turned to the incident rays, and the white side be viewed 
from behind, by using an open frame. But in this mode of observing, a strong light must be thrown on the 
white side, to enable the spectator to see and measure the effect ; and at the same moment the spectrum must 
be intercepted, which is inconvenient unless an assistant be present. 
