110 =©Sir J. F. W. Herschel on the Action of the Rays 
increase, and is strong at + 23, where the maximum of effect 
is situated. Hence it degrades more slowly, is still pretty 
strong at + 60, and may be traced as far as 80, being there- 
fore nearly commensurate with the spectrum impressed on 
nitro-argentine paper, a range of action unique, so far as 
my experience goes in vegetable photography. ‘The species 
experimented on is that which (supposing it undescribed) I 
should be disposed to call ¢riangularis, from the angular sec- 
tion of its long, slender, smooth, solid leaves; which, with 
the singular character of its juice, may serve to identify the 
species, my own specimen (a single one) having been destroyed 
by insects after flowering superbly. The ultimate tint ac- 
quired by the juice is a deep brown, to which it also passes in 
darkness, but much more slowly. The juices of both species, 
however, have the same photographic characters. 
191. Chetranthus cheiri, Wall-flower.—A cultivated double 
variety of this Hower, remarkable for the purity of its bright 
yellow tint, and the abundance and duration of its flowers, 
yields ajuice when expressed with alcohol, from which subsides, 
on standing, a bright yellew, uniform, finely divided frecula, 
leaving a greenish yellow transparent liquid, only slightly co- 
loured, supernatant. ‘The feecula spreads well on paper, and 
is very sensitive to the action of light, but appears at the same 
time to undergo a sort of chromatic analysis, and to comport 
itself as if composed of two very distinct colouring principles, 
very differently affected. The one on which the intensity and 
sub-orange tint of the colour depends is speedily destroyed, 
but the paper is not thereby fully whitened. A paler yellow 
remains as a residual tint, and this, on continued exposure to 
light, so far from diminishing in tone, slowly darkens to brown. 
Exposed to the spectrum, the paper is first speedily reduced 
nearly to whiteness in the region of the blue and violet rays. 
More slowly, an insulated solar image is whitened at — 10°5, 
or in the less refrangible portion of the red, and the impressed 
spectrum assumes the type represented in fig. 7, where m Y 
= — 10°5; m’ Y= + 13:0; Yc = + 55. The exposure 
continuing, a brown impression begins to be perceived in the 
midst of the white streak, which darkens very slowly from 
+186 to + 42. It never attains any great intensity, but 
presents a singular appearance in the midst of the white train 
previously eaten out. 
192. ‘Lhe juice in question contains gallic acid, and pro- 
bably tannin, as is evident from its striking a strong black 
with persalts of iron. ‘The gallic acid itself (whose singular 
properties, in conjunction with nitrate of silver, have been de- 
veloped by Mr, Talbot, as the basis of his all but magical 
