28 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
terminates the spectrum at + 90’23, making 1 the total length of the darkened or dis- 
coloured impression 97'83 parts. 
75. If there be dispersed light, the spectrum is lengthened out by a pinkish white 
addition extending below the fiducial point as far as — 22'43 ; so that the whole 
extent of the action on this paper is 112‘66, being considerably more than double the 
total length of the ordinary luminous spectrum*. 
76. It should be observed that, as respects the colours, the nature of the paper itself 
is far from indifferent. The thin paper, for example, manufactured in China, on which 
the books of that country are printed, is in many respects peculiar in its photographic 
properties, and in none more than in the comparative absence of the red and strength 
of the green tints produced on it, when subjected to the preparation of alternate 
washes of nitrate of silver and salt. The length, however, of the total spectrum is 
the same, but owing to the absence of the terminal red fringe, the darkened portion 
does not extend more than a semidiameter of the sun below the fiducial centre (thus 
marking a limit of action at that precise point), and the light portion is very feeble 
and indistinct. 
Chemical Action traced much beyond the extreme Red Rays. — Exercise of a Blackening 
or deoxidating Power by the Red Rays themselves, under certain circumstances. 
77- Hydrobr ornate of Potash. (Paper No. 571.) — Paper prepared with this salt 
instead of muriate of soda, gives a most extraordinary spectrum. The action com- 
mences, the moment the rays fall upon it, visibly over its whole length, and is uniform 
in intensity at every point except just at the extremities, where of course it fades gra- 
dually. But the most characteristic peculiarity of this spectrum is its extravagant 
length. Instead of terminating at the fiducial point, or thereabouts, the darkened 
portion extends down to the very extremity of the visible red rays. In tint it is pretty 
uniform (a grey-black not by any means intense) over the whole length, except that 
a slight fringe of redness (but no green or blue) is perceptible at the least refracted 
end. Beyond these evidences of deoxidation, thus traced fully up to the visible limit 
of the spectrum, it might be expected that all photographic action would terminate. 
Far otherwise ; a contrary or oxidizing action now commences, marked by the main- 
tenance of whiteness on the paper under the influence of dispersed light, and extends 
to a distance so considerable below the last traceable limit of the luminous spectrum, 
as to leave no possible room for mistake or doubt that chemical powers are exerted 
over a large extra-spectral region in this direction. The following statement of di- 
mensions, a mean of three experiments, will place this important proposition in the 
clearest evidence. The measures are, as usual, from the fiducial point or centre of 
the yellow ray. 
* See further on, Art. 77. 
