of the Solar Spectrum on Vegetable Colours. 111 
process of the calotype*) is affected also negatively by light. 
Paper washed with its spirituous solution and partially co- 
vered, 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 phaenomenon above 
recorded. It would rather appear as if some portion of a 
more decidedly negative ingredient analogous to that which 
exists in the Bulbine, were present. As regards the positive 
ingredient, | may mention here the common Marigold (in 
which also the colour resides in an insoluble feecula) as 
a flower in which the colouring principle is probably iden- 
tical 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 Mimulust, 
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 yel- 
low stain which agrees in sensibility, and in the type-of the 
impressed spectrum with the Corchorus. The Ferranea un- 
dulata, 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 prin- 
ciple, which, mingling with 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 the colour of the American Marigold, 
causes its tint to pass rapidly in sunshine from brown to green, 
after which continued exposure produces no further change. 
The yellow colour of fresh bees’-wax and of palm-oil, are 
also, I doubt not, referable to the same, or a nearly similar 
* Preparations of the gallic acid in conjunction with silver, are noticed 
by me in my former paper as forming a “ problematic exception” to my 
general want of success in procuring at the very outset of my photogra- 
phic ey ments (in February 1839), papers more sensitive than the simple 
nitrated or carbonated ones. ‘The problematic feature consisted in spon- 
taneous 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, 
+ Mimulus Smithii (Lindl,), t French Marigold, Tagetes Patula. 
