RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 51 
]S T 0 tc I. — On the Distribution of the Calorific Rays in the Solar Spectrum. 
Received March 4, — Read March 5, 1840. 
124. Since the communication to the Royal Society, of the foregoing pages, I have 
discovered a process by which the calorific rays in the solar spectrum are made to 
leave their impress on a surface properly prepared for the purpose, so as to form what 
may be called a thermograph of the spectrum, in which the intensity of the thermic 
ray of any given refrangibility is indicated by the degree of whiteness produced on a 
black or very dark ground, by the action of the ray at the point where it is received 
at the surface. From the nature of the process it is demonstrable, that only rays of 
heat, and not those which are usually called chemical, can be concerned in the effect 
produced. 
125. It is easy to see what a variety of interesting applications this lays open to 
those engaged in studying the subject of transcalescence, whether in its relations to 
coloured or colourless media. As a specimen of what may be expected from the mode 
of observation in question, I shall proceed to describe the thermograph of a spectrum 
transmitted, 1st, through the prism of Fraunhofer’s flint-glass, described in Art. 57 
of the foregoing paper ; and 2ndly, through the flint- and crown-lenses of the achro- 
matic object-glass, there also mentioned ; premising, however, that the observations 
were made in occasional and by no means powerful gleams of sun, in a day generally 
and densely cloudy. 
126. Recurring to the dimensions and signs of the former articles, used in de- 
scribing the chemical and luminous spectra, the thermograph in question consists of 
a white streak of the breadth of the sun’s diameter, extending, where broadest and 
best defined, towards the positive or more refrangible end of the spectrum, to about 
+ 25 or 30 parts, (of which the old Newtonian spectrum, taking in the extreme 
visible colours, occupies 54,) but fading away with great rapidity, and at the point 
so indicated no longer traceable, a point corresponding to about the end of the 
brighter blue rays. On the negative or less refrangible side of the fiducial yellow, 
however, it goes on increasing to a maximum at — 16, that is to say, 3"2 parts be- 
yond the extreme red ray. (The dimensions here referred to are reduced to a linear 
spectrum, or to one formed by a luminary having no sensible diameter.) Beyond 
this maximum, the intensity of whiteness decreases to a minimum at — 31, where it 
is nearly evanescent, but immediately again recovers, so as to form a faint but un- 
equivocal whitened solar image, insulated from the rest of the thermic spectrum, whose 
centre is situated at — 35 ; thus imitating, in a certain sense, the coloured spectrum 
of the cobalt-glass, in which a faint and otherwise indiscernible red ray is insulated 
from the rest of the coloured spectrum. 
12/. If any doubt should remain in the minds of photologists as to the correctness 
of my father’s results, which placed the maximum of heat beyond the last visible red. 
