8 Sir J. F. W. Herschel on the Action of the Rays 
dinate agreeably to Art. 93, where it is shown that these rays 
not only prevent the blue colour from being produced by the 
more refrangible ones, but destroy it when so produced. 
Another specimen gave the following dimensions : Ya=-— 
11:4Y b = —9°5, Yc=+30°0, Yd = +61°0, Ye = +80°4, 
and this is the greatest extent of action I have hitherto ob- 
served. 
152. A portion of the same paper was exposed, dry, to an 
atmosphere of chlorine considerably diluted with common air, 
which imparted to it a pale, dirty, greenish yellow hue. Being 
thence transferred immediately to the spectrum, the result 
was nota little remarkable. The whole spectrum, the green 
excepted, was impressed in faint tints nearly corresponding 
to the natural ones. ‘The red was evident—the yellow dilute 
and nearly white—the blue a fine sky-blue, while beyond the 
violet succeeded a train of somewhat greenish darkness. ‘These 
tints proved fugitive, and in twenty-four hours were nearly 
obliterated. 
153. When paper fresh washed with tincture of guaiacum 
and stili wet is exposed to chlorine, it instantly acquires a fine 
and full Prussian blue colour, which however passes speedily 
to brown if the action be prolonged. The colour is difficult 
to preserve in its full intensity, and fades considerably in dry- 
ing, becoming at the same time somewhat greenish. Exposed 
wet to the spectrum, it is found to have become much more 
sensitive, and is immediately attacked with great energy by 
the red rays, which destroy the blue colour, converting it to 
a brownish or reddish yellow. The action extends rapidly up 
the spectrum as far as the extreme violet, in which ray, how- 
ever, the tint impressed or left undestroyed passes to a hue 
partaking of violet, and indicating by the change what ought 
probably to be regarded as a neutral point at +12°0. The 
impressed spectrum (corrected for semidiameter) commences 
at a, fig. 2, at —13°4; the maximum 6 of the positive action 
occurs at —9°0, the neutral point c at +12°0, the maximum 
d of negative action at +33'0, and the sensible termination e 
of the impression at + 60:0. 
154. The action of gaseous chlorine is too energetic to be 
easily arrested at the proper point, besides which this gas also 
acts powerfully on the alcohol employed. To obviate these 
inconveniences, paper thoroughly impregnated with guaiacum 
by washing with the tincture, and drying in a gentle heat, was 
steeped in weak aqueous solution of chlorine, by which pro- 
cess it slowly acquired a beautiful and pure celestial blue co- 
lour. It is very sensitive, and may be conveniently used for 
copying engravings, &¢c., which it does with this singularity, 
