RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 35 
Its commencement and termination were, however, very feeble, graduating' off insen- 
sibly; but at the maximum, which occurred a little below the fiducial point, (corre- 
sponding nearly with the orange rays of the luminous spectrum,) the blue colour was 
completely discharged. Beyond the violet there was no indication of increase of 
colour, or of any other action. I do not find that this paper is discoloured by mere 
radiant heat unaccompanied with light. 
92. Paper washed with an alcoholic solution of guaiacum was exposed to the spec- 
trum formed by the Fraunhofer flint prism, and concentrated as usual. The paper 
before exposure was of a very pale yellowish colour. The action was slow, (though 
much quicker than in the case of the tincture of heartsease,) and extended from + 47, 
or thereabouts, to -f- 72‘9, attaining its maximum at + 58'5, or beyond the violet, far 
in upon the lavender rays. The impressed spectrum throughout its whole length 
(68'2 parts) was of a fine celestial blue colour, though of little intensity. 
93. A slip of the same paper was exposed for some time to sunshine, defended from 
the action of all but the blue and violet rays by cupro-sulphate of ammonia, acquiring 
thereby a uniform pale blue tint, verging to greenish. It was then exposed to the 
spectrum as in the last article. The effect was highly remarkable : the impression 
produced consisted of two long very faint streaks, the one, corresponding to the more 
refrangible part of the spectrum, from about -f 42 to + 68, was of a blue colour, 
darker than the general ground, and having its maximum at nearly the same point 
as in the last experiment, viz. at + 58. The other, corresponding to the compara- 
tively less refrangible portion of the total spectrum, commenced above at about + 27, 
as nearly as the extreme faintness would enable the eye to seize its beginning, and 
extended downwards to — 16. In this portion the blue colour of the ground of the 
paper was discharged more or less completely, and most so (where the original pale 
yellow colour of the paper was quite restored,) at — 3, that is to say upon the orange 
ray, and by no means at that point of the spectrum where the caloric effect is greatest*. 
The effect therefore cannot be due to mere radiant heat. The neutral part from + 2 7 
to + 42, embraces the whole of the violet and nearly all the indigo rays of the lu- 
minous spectrum. 
V. Of the Whitening Power of the several Rays of the Spectrum under the influence 
of the Hydriodic Salts on Papers variously prepared and previously darkened by 
the action of Solar Light. 
94. The singular effect of a moderately strong application of the hydriodate of 
potash to darkened photographic papers, in rendering them susceptible of being 
whitened by further exposure to light, has been already alluded to. But the pris- 
matic analysis of this effect, as exhibited on various preparations of silver, offers a 
series of new relations so very curious, as may well require the bestowal on them of 
* See the Notes at the end of this paper. 
F 2 
