Action of the Rays of the Solar Spectrum. 335 
mical action. This branch of the inquiry was suggested by the fact, 
noticed by the author in his former communication, that the dark- 
ening power of the solar rays was considerably increased by the in- 
terposition of a plate of glass in close contact with the photographic 
paper. The influence of various other media, superposed on pre- 
pared paper, was ascertained by experiment, and the results are re- 
corded in a tabular form. 
14. The paper concludes with the description of an Actinograph, 
or self-registering Photometer for meteorological purposes : its ob- 
jects being to obtain a permanent and self-comparable register and 
measure, first, of the momentary amount of general illumination in 
the visible hemisphere, which constitutes day-light ; and secondly, 
of the intensity, duration, and interruption of actual sunshine, or, 
when the sun is not visible, of that point in the clouded sky behind 
which the sun is situated. 
In a postscript, dated March 3rd, 1 840, the author states that he 
has discovered a process by which the calorific rays in the solar spec- 
trum are made to affect a surface properly prepared for that pur- 
pose, so as to form what may be called a thermograph of the spec- 
trum ; in which the intensity of the thermic ray of any given refran- 
gibility is indicated by the degree of whiteness produced on a black 
ground, by the action of the ray at the points where it is received 
at that surface, the most remarkable result of which is the insula- 
tion of heat-spots or thermic images of the sun quite apart from the 
great body of the thermic spectrum. Thus the whole extent over 
which prismatic dispersion scatters the sun’s rays, including the 
calorific effect of the least, and the chemical agency of the most re- 
frangible, is considerably more than twice as great as the Newtonian 
coloured spectrum. 
In a second note, communicated March 12*, 1840, the author de- 
scribes his process for rendering visible the thermic spectrum, which 
consists in smoking one side of very thin white paper till it is com- 
pletely blackened, exposing the white surface to the spectrum and 
washing it over with alcohol. The thermic rays, by drying the 
points on which they impinge more rapidly than the rest of the sur- 
face, trace out their extent and the law of their distribution by a 
whiteness so induced on the general blackness which the whole sur- 
face acquires by the absorption of the liquid into the pores of the 
paper. He also explains a method by which the impression thus 
made, and which is only transient, can be rendered permanent. 
This method of observation is then applied to the further exa- 
mination of various points connected with the distribution of the 
thermic rays, the transcalescence of particular media, the polari- 
zation of radiant heat (which is easily rendered sensible by this me- 
thod), &c. The reality of more or less insulated spots of heat dis- 
tributed at very nearly equal intervals along the axis of the spec- 
trum (and of which the origin is probably to be sought in the flint 
glass prism used — but possibly in atmospheric absorption) is esta- 
blished. Of these spots, two of an oval form, are situated, the one 
nearly at, and the other some distance beyond the extreme red end 
