Chemical Force in the Spectrum. 35 
impression obtained: 1st, when eit light has been 
excluded ; and 2d, when it has been permitted simultaneously 
or previously to act. 
n the latter case, in all that region of the spectrum from the 
more refrangible extremity to somewhat below the line G, 
the usual darkening effect manifested by silver compounds is 
observed, but beyond this, and to the extreme less refrangible 
rays, with certain variations of intensity, the action of the 
extraneous and simultaneously acting light is checked, and the 
effect of previously acting light is destroyed. 
It happened that in 1842 I obtained two very fine specimens 
of the latter spectra; one of these I sent to Sir J. Herschel, the 
other is still in my possession. 
In the Philosophical Magazine (Feb., 1848), Herschel gave a 
detailed description of these spectrum impressions. He was 
isposed to refer the appearance they present to the phenomena 
of thin films, but at the same time pointed out the difficulties 
im the way of that explanation, He also sent me three roofs 
he had cbinined on ordinary sensitive paper, darkened ry ex- 
Posure to light, then washed with a solution of iodide of te - 
stum, and placed in the spectrum. He described them as follows: 
chant paper from which excess of nitrate of silver 
has not been abstracted, under the influence of an iodic salt, 
Produced by a November sun. N. B.—View it also trans- 
parently against the light.” 
