Chemical Force in the Spectrum. 27 
The general appearance of the photograph at once suggests 
that the less refrangible rays can arrest the action of the day- 
light, and protect the silver iodide from change. A close ex- 
amination shows that there are three points, the extreme red, 
the center of the yellow, and the extreme violet, which ap- 
parently can hold the daylight in check. There are also two 
intervening ones in which the actions conspire. The point of 
maximum protection corresponds to the point of maximum 
action referred to above in paragraph (1). 
(3.) If the metallic tablet, previously to its exposure to the 
spectrum, be submitted for a few moments to a weak light, so 
that were it developed it would at this stage whiten all over, 
the action of the spectrum upon it will be the same as in the 
last case (2). But this change in the mode of the experiment 
leads to a very important conclusion. The less refrangible rays 
can reverse or undo the change, in whatever it may consist, that 
light has already impressed on the iodide of silver. ee 
ow bearing in mind the facts, that the photographie action 
of diffused light on this iodide is mainly due to the more re- 
rangible rays it contains, we are brought by these experiments 
to the following conclusions : 
Ist. Every ray in the spectrum acts on silver iodide. 
2d. The more refrangible rays apparently promote the action 
of the daylight on that substance ; the less refrangible apparently 
arrest 1t. : 
ing substance of the silver iodide are involved. 
[ abstain for the moment from giving further details of these 
ie Herschel, in the case of one I sent him many years ago. 
His examination of it, illustrated by a lithograph, may be found 
im the Philosophical Magazine (Feb., 1843). I shall have to 
return to the subject of the behavior of silver iodide in presence 
of radiations on a subsequent page of this memoir. : 
_ The main point at present established is this, that the silver 
iodide under proper treatment is affected by every ray that a 
flint -glass prism can transmit, and therefore it Is altogether 
erroneous to suppose that chemical force is restricted to the 
more refrangible portions of the spectrum. 
2d.—Case of Bitumens and Resms. 
These substances are of special interest in the history of pho- 
tography, since in the hands of Niéped they probably were the 
