10 ~+Sir J. F. W. Herschel on the Action of the Rays 
consideration of his experiments and of those described in 
the last article, that nothing further is requisite for operating 
the change from the green or blue to the yellow state, than 
the assumption of a certain temperature dependent on its state 
of dryness, and varying according to that state between the 
limits of 180° and 280°. Nevertheless, if we consider that 
the same change is produced by rays of the spectram which 
are very far from being the hottest, while yet the extra-spectral 
thermic rays, under precisely the same circumstances of ex- 
posure, produce no such effect, though far surpassing in 
mere calorific power those which do, we shall see reason to 
doubt the sufficiency of this view of the matter. The follow- 
ing experiments were therefore instituted with a view to its 
further elucidation. 
158. A slip of the paper No. 1168 was moistened and sub- 
jected in clear sunshine to the action of the spectrum. ‘The 
colour was discharged from the region occupied by the less 
refrangible luminous rays, as described in Art. 185. At the 
same time, the more distant thermic rays beyond the spectrum 
produced their proper effect, in evaporating the moisture from 
those portions on which they fell; so that in due time the 
heat-spots § and y became apparent (see Art. 186.), the former 
very distinctly, the latter perceptibly. The spot 6 (which is 
remarkable) was scarcely if at all formed. So long then as 
the paper continued moist and remained under the influence 
of the thermic rays, the appearances were those of a diminu- 
tion of colour (Art. 1$1.), operated by the thermic rays 8 
and y. But the discoloration in these points was only ap- 
parent, for as the paper dried these heat-spuis disappeared, 
leaving its colour quite unchanged at those points; while the 
photographic impression really produced within the visible 
spectrum, remained and went on increasing in intensity. The 
non-luminous thermic rays, therefore, though clearly shown 
to have been active as heat, were yet incapable of effecting that 
peculiar chemical change which other rays much less copiously 
endowed with heating power, were all the while producing. 
159. It may be objected to this, that no proof is afforded 
in the above-related experiment, that any part of the paper 
actually attained a temperature of 180° or more; that in cone 
sequence no discoloration due to the action of heat (quoad 
heat) was produced; and that the discoloration which did 
take place was swi generis, and originated with the light and 
not the heat of that part of the spectrum to which it corre- 
sponded. A slip of the same paper (1168.) was therefore ex- 
posed dry to the spectrum in such a way as to leave its back 
accessible; and an iron heated below redness was then ap- 
