40 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
more refrangible end. And to these indications the photographic spectrum was found 
to correspond, being reduced to an oval patch not exceeding nineteen parts in extent, 
between the limits + 135 and — 5-5, and that, it should be observed, of a uniform 
tint, the red fringe at the least refracted end (which was pretty conspicuous on the 
paper used when subjected to the unimpeded spectrum) being deficient, from which 
it may be surmised that this liquid acts more powerfully on the chemical rays at the 
red end of the spectrum than on the luminous. The contrast between the complete 
and absorbed spectra in this experiment was indeed most striking, considering the 
small apparent loss of illuminative power. The unabsorbed spectrum extended from 
— 8’0 to + 87, or over ninety-five parts, throughout fully sixty-five of which its in- 
tensity was uniform, and its hue deep black. A like degree of blackness extended 
only over thirteen parts of the absorbed spectrum, so that while the illuminating 
power of an incident sunbeam was so little impaired by the action of the liquid as 
already stated, its total photographic effect must have been diminished by the loss of 
at least four fifths of its amount. It should also be mentioned that in this experiment 
there was not the least trace of the action being resumed in the region beyond the 
violet rays. 
106 . The singular properties of nitrous gas pointed out by Sir David Brewster as 
exhibited in its absorbent action on the spectrum, which it breaks up through its 
whole extent into alternating bright and dark spaces, led me to expect some equally 
singular affection of the photographic spectrum after undergoing its absorption. But 
this expectation was not fulfilled. The photographic impression was reduced by the 
action of a glass globe filled with this gas, to a small extent (twenty-five parts) by the 
destruction of all action beyond the blue rays ; but in what remained I could discern 
no trace of interruptions of continuity. The experiment, I should observe, however, 
was difficult, having no means of drying the gas at hand, so that to prevent the de- 
position of moisture the globe was obliged to be frequently heated. It would be de- 
sirable therefore that it should be repeated, and that other absorbent gases should 
also be tried. 
107. Another and no less striking example of the independence of the laws of ab- 
sorptive action of media as exerted on the chemical and luminous rays, is afforded 
by the cupro-sulphate of ammonia. That beautiful liquid, as I have shown in my 
paper above referred to*, acts with the greatest energy on the red end of the spec- 
trum, which it completely obliterates in a very small thickness, the action diminishing 
progressively till it seems reduced almost to nothing at the more refrangible extremity. 
Its scale of diacratescence, however, is quite different from what this indication would 
lead us to expect. The absorptive action on the chemical, as on the luminous rays 
indeed, at the less refrangible end, is intense, and goes on diminishing progressively 
to about the middle of the indigo, but here it begins again to be reinforced, and at- 
tains an energy in the region of the extreme violet, and over the whole extent of the 
* Transactions of the Royal Society of Edinburgh, 1822. 
