OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
197 
lated solar image is whitened at — 10 - 5, or in the less refrangible portion of the red, 
and the impresssd 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 
4- 18 - 6 to +42. It never attains any great intensity, but presents a singular ap- 
pearance in the midst of the white train previously eaten out. 
192. The juice in question contains gallic acid, and probably 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 developed by Mr. 
Talbot, as the basis of his all but magical process of the calotype*) is affected also 
negatively by light. Paper washed with its spirituous solution and partially covered, 
being exposed several months in a window, was found pretty strongly darkened in all 
the exposed portion. The action is too slow for prismatic analysis, and I am far 
from attributing to the presence of this acid the phenomenon above recorded. It 
would rather appear as if some portion of a more decidedly negative ingredient ana- 
logous to that which exists in the Bulb me ^ were present. As regards the positive 
ingredient, I may mention here the common Marigold (in which also the colour 
resides in an insoluble fsecula) as a flower in which the colouring principle is probably 
identical both with this and with that of the Corchorus Japonica, since it comports 
itself in the very same manner under the spectrum, — is nearly, or quite as sensitive, 
and is moreover fugitive, even when carefully defended from light, giving photographs 
which cannot be preserved. Many other flowers also contain in their juices a portion 
of this identical, or a very similar yellow principle, probably in a state of greater 
solubility, and thence disposed to the absorption of oxygen. Thus the juice of a fine 
purely yellow species of Mimulus -j-, if expressed, with or without alcohol, though 
vividly yellow in the first moments of expression, passes almost instantly to dirty 
green, and loses its sensibility to light ; but if crushed on paper and immediately 
dried, the petals give a bright yellow stain which agrees in sensibility, and in the 
type of the impressed spectrum with the Corchorus. The Ferranea undulata , a dark 
brown flower, yields, when expressed, a dull green juice, which, spread on paper and 
dried, turns very speedily blue under the influence of the blue and violet rays of the 
spectrum ; owing to the destruction of this yellow principle, which, mingling with the 
the substratum of blue (itself a much more indestructible tint), gives it its natural 
tinge of green. A similar destruction, of probably again the same yellow matter, in 
* Preparations of the gallic acid in conjunction with silver, are noticed by me in my former paper as form- 
ing a “ problematic exception” to my general want of success in procuring at the very outset of my photogra- 
phic experiments (in February 1839), papers more sensitive than the simple nitrated or carbonated ones. The 
problematic feature consisted in spontaneous darkening of the papers laid by to dry in the dark, so at least 
then considered, but really arising doubtless from light incident on them in their preparation. Acetate of 
silver was used in their preparation. 
f Mimulus Smithii (Lindl.). 
