14 Sir J. F. W. Herschel on the Action of the Rays 
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 requisite 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 generally be found successful. ‘The paper 
should be moistened at the back by sponging and blotting off. 
It should then be pinned on a board, the moist side down- 
wards, 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 ¢o pass clearly 
over those which project, and observing also to carry the tint 
from below upwards 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 
alcohol 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 move- 
ment 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. 
Ist. 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 colour are separated, by destroying the one and 
leaving the other outstanding. The older the paper, or the 
tincture with which it is stained, the greater is the amount of 
this residual tint. 
