of the Solar Spectrum on the Daguerreotype Plate. 127 
In reference to the “protecting” rays of Dr. Draper, 
assuredly the first aspect of the phenomena is favourable to 
their existence; yet I must observe, that when the spectrum 
is made to act on ioduretted paper in the mode described in 
the memoir above referred to, the general ground of the 
paper is affected by the dispersed light in the same manner 
but to a less degree than the spectrum-portion, nor is there 
any sign of the occurrence about the region of the extreme 
red and beyond it of that sort of protection from the action 
of dispersed light which I have described in Phil. Trans, 
1840, and to which Dr. Draper refers, as due to the action 
of negative rays stated by me to exist in that region. I have 
however (as will be seen on referring to Article 90 of that 
paper) been very cautious in ascribing to them any such de- 
cidedly negative quality. An effect of some kind, conservative 
of the whiteness of papers variously prepared while under 
the influence of dispersed light, does undoubtedly take place 
in the region of the extreme red, and to a certain distance 
below it. But I have of late learned to regard that effect 
(which at first Igconfess had very much the appearance of a 
negative action) as at least in part a secondary one, and due 
to acause which has no existence when silver plates are used, 
viz. the maintenance of the paper at those particular parts 
in a state of superior dryness to the surrounding parts, by 
which its sensibility is materially diminished—a certain slight 
degree of moisture being exceedingly favourable to photo- 
graphic action in many cases, as | especially find it to be in 
that of muriated papers overkept, in which the conservative 
effect is occasionally so remarkable as to extend over the whole 
visible or luminous spectrum. And in one instance (recorded 
in Article 95 of that paper) we have an instance very much 
in point, and the more valuable as having been marked as an 
anomaly at the time, while yet the phenomena of the thermic 
spectrum, as manifested in the drying of paper, were unknown 
to myself. For in that article it is recorded that in a certain 
paper (which had it appears been kept some time before 
using) the conservative action in question was limited to a 
circular well-defined solar image beyond the extreme red, and 
was due, I have now no doubt, to the action of the heat-spot 
B (see Article 136) either drying the paper, or exerting, it 
may be, some peculiar effect due to the chemical nature of its 
heat. For I would wish to be understood as by no means 
limiting the cause of conservation to this secondary mode of 
action, or to deny the exercise of an influence which may so 
Jar be regarded as negative, i. e. as opposed to photographic 
agency by that portion of the ¢hermic spectrum which lies 
