return the plate to
the bath, to obtain the values of the other distances. The temperature
has a great effect on the intensity of the ebullitions, and it is hardly
possible to depend on it absolutely as a fixed basis on which to rest a
calculation of the time necessary for each biting, as its own
variability renders it difficult to appreciate the aid to be received
from it. In winter, for instance, with the same strength of acid, it
needs four or five times as much time to reach the same result as in
summer, so that on very hot days the biting progresses so rapidly that
the plate cannot be lost sight of for a single moment without risk of
over-biting.

[Illustration: Pl. I_a_.]

35. =Biting continued.=--We have now obtained several moderate
ebullitions, and as it would not do to exaggerate the tone of the
mountain in the background, it is time to withdraw the plate once more.
Uncover a single line by removing the ground, either with the nail of
your finger or with a very small brush dipped into spirits of
turpentine, to examine whether it is deeply enough bitten for the
distance which it is to represent. If the depth is not sufficient, cover
it with stopping-out varnish, and bite again. This is not necessary,
however, in our present case, and you may therefore stop out the whole
background. Remember, if you please, that the line must look _less_
heavy than it is to show in the proof; for you must take into account
the black color of the printing-ink. With your brush go over the edges
of the trees which are to be relieved rather lightly against the sky, as
well as over that part of the shadow in this tower which blends with the
light. There are also some delicate passages in the figure of the woman
in the foreground, in the details of the plants, and in the folds of
this tent (Pl. I_a_). Stop out all these, and do not lose sight of the
values of the original (Pl. II.). Make use of the brush to revarnish
several places which are scaling off on the margin and the back of the
plate. The temperature is favorable; the ebullitions come on without
letting us wait long, and the plate is bluing rapidly. I do not like to
see these operations drag on; in winter, therefore, I do my biting near
the fire. We soon acquire a passion for biting, and take an ever-growing
interest in it, which is incessantly sharpened by thinking of the result
to which we aspire. Hence the desire of constant observation, and that
assiduity in following