178 Sir J. F. W. Herschel on the Action of the Rays 
certain that what little action of the kind exists is due to the 
effect of casual dispersed light incident in the preparation of 
the paper. I have before me a specimen of paper so treated, 
in which the effect of thirty seconds exposure to sunshine was 
quite invisible at first, and which is now of so intense a purple 
as may well be called black, while the unsunned portion has 
acquired comparatively but a very slight brown. And (which 
is not a little remarkable, and indicates that in the time of 
exposure mentioned the maaimum of effect was attained) other 
portions of the same paper exposed in graduated progression 
for longer times, viz. 1™, 2™, and 3™, are not in the least per- 
ceptible degree darker than the portion on which the light 
had acted during thirty seconds only. 
214. The very remarkable phenomenon, described in Art. 
208. of a second darkening, different in character and colour, 
coming on after the bleaching effect of solar light has been 
fully completed, is not without a parallel among the argentine 
compounds. I refer to the action of the hydriodic salts on argen- 
tine papers completely blackened by exposure to sunshine, an 
action imperfectly described in § 5. of my former paper (Art.94 
et seq.), and signalized as to one of its most striking peculia- 
rities in Note 2, Art. 129. of that communication. To study 
the phznomena of this action in their simplest form, a paper 
prepared without iodine, and of a positive character is re- 
quired. The simplest and most convenient is that prepared 
by Mr. Hunt with one wash of muriate of ammonia, two of 
nitrate of silver, and exposure to sunshine. With such paper 
(obligingly furnished me by Mr. Hunt himself) I made the 
following experiments. 
215. Exposed to the spectrum and washed with a solution 
of hydriodate of potash too weak fully to excite it +, two con- 
trary actions were produced by the rays above and below the 
zero point or mean yellow. By the former the paper began 
to be bleached at a point distant + 26°5 parts from the zero, 
from which point the bleaching extended gradually upwards to 
a considerable distance, and downwards to the circumference 
of a semicircle, having that point for a centre. By the latter 
the paper was darkened (at least in comparison with its general 
surface, which, purposely subjected to dispersed light, had be- 
gun to lose much of its original intense blackness), the darkness 
spreading also upwards and downwards: upwards till it passed 
the zero point, and nearly or quite attained the semicircle above 
mentioned ; and downwards to about — 19, or — 20 parts. As 
* Muriate of ammonia forty grains, water four ounces ; nitrate of silver 
sixty grains, water one ounce. 
+ Ioduret of potassium sixty grains, water one ounce, 
