188 
SIR J. F. W. HERSCHEE ON THE ACTION OF THE RAYS 
167- This change of colour is probably owing to different causes in different 
flowers. In some it undoubtedly arises from the escape of carbonic acid, but this as a 
general cause for the change from red to blue, has, I am aware, been controverted*. 
In some (as is the case with the yellow Ranunculi) it seems to arise from a chemi- 
cal alteration depending on absorption of oxygen ; and in others, especially where 
the expressed juice coagulates on standing, to a loss of vitality or disorganization of 
the molecules. The fresh petal of a single flower, merely crushed by rubbing on dry 
paper, and instantly dried, leaves a stain much more nearly approximating to the 
original hue. This, for example, is the only way in which the fine blue colour of the 
common field Veronica can be imparted to paper. Its expressed juice, however 
quickly prepared, when laid on with a brush, affords only a dirty neutral gray, and 
so of many others. But in this way no even tint can be had, which is a first requi- 
site to the experiments now in question, as well as to their application to photography. 
168. To secure this desirable evenness of tint, the following manipulation will ge- 
nerally be found successful. The paper should be moistened at the back by spon- 
ging and blotting off. It should then be pinned on a board, the moist side downwards, 
so that two of its edges (suppose the right-hand and lower ones) shall project a little 
beyond those of the board. The board being then inclined twenty or thirty degrees 
to the horizon, the alcoholic tincture (mixed with a very little water, if the petals 
themselves be not very juicy) is to be applied with a brush in strokes from left to 
right, taking care not to go over the edges which rest on the board, but to pass 
clearly over those which project, and observing also to carry the tint from below up- 
wards by quick sweeping strokes, leaving no dry spaces between them, but keeping 
up a continuity of wet surface. When all is wet, cross them by another set of strokes 
from above downwards, so managing the brush as to leave no floating liquid on the 
paper. It must then be dried as quickly as possible over a stove, or in a current of 
warm air, avoiding, however, such heat as may injure the tint. The presence of al- 
cohol prevents the solution of the gummy principle, which, when present, gives a 
smeary surface ; but the evenness of tint given by this process results chiefly from 
that singular intestine movement which always takes place when alcohol is in the act 
of separation from water by evaporation — a movement which disperses knots and 
blots in the film of liquid with great energy, and spreads them over the surrounding 
surface. 
169. The action of the spectrum, or of white light, on the colours of flowers and 
leaves, is extremely various, both as regards its total intensity and the distribution of 
the active rays over the spectrum. But certain peculiarities in this species of action 
obtain almost universally. 
1st. The action is positive , that is to say, light destroys colour; either totally, or 
leaving a residual tint, on which it has no further, or a very much slower action. 
And thus is effected a sort of chromatic analysis, in which two distinct elements of 
* Nicholson’s Journal. 
