Chemical Force in the Spectrum. 37 
In 1847 M. Claudet communicated a paper to the Royal 
Society, subsequently published in the Philosophical Magazine 
(Feb., 1848), on this subject. His attention had been drawn to 
it by observing that the red image of the sun, during a dense 
fog, had destroyed the effect previously produced on a sensi- 
tive silver surface, and that this destruction could be occasioned 
at pleasure by the use of red and yellow screens. surface 
ich has been impressed by daylight, and the impression 
then obliterated by the less refrangible rays, had recovered its 
primitive condition. It was ready to be impressed again b 
daylight, and again the resulting’ effect might be destroyed. 
Claudet found that this excitation and neutralization might 
be repeated many times, the chemical constitution of the film 
remaining unchanged to the last. 
These facts seem to be inconsistent with Herschel’s opinion, 
that positive and negative pictures may succeed each other b 
the continued action of a radiation, on the principle of Newton's 
ri 
not a trace of action can be detected. The lines a, f, v, can- 
not be obtained on collodion. There is, therefore, a difference 
between its behavior under exposure to light and that of a 
daguerreotype tablet. 
The reversals that are obtained on collodion by the use of 
certain haloid compounds, are altogether different from the re- 
Versals on the thin films of a silver tablet. They are produced 
by the more refrangible rays. 
On exposing a collodion surface prepared in the usual man- 
ner to daylight, long enough to stain it completely, then wash- 
Ing off the free nitrate, and in succession dipping the plate into 
a weak solution of iodide of potassium, exposing it to the spec- 
trum, washing, again dipping it in the nitrate bath, and finally 
developing, a reversed action is obtained. The daylight is per- 
fectly neutralized, but not after the manner in a spp toneegee 
sure of five seconds. In twelve seconds the protected space is 
much larger; in thirty seconds it has spread from F to H. Its, 
however, to be particularly remarked that the less refrangible 
Tays show no action. : Es 
The results are substantially the same when, instead of iodide 
of potassium, the chloride of sodium, corrosive sublimate, bro- 
