within the visible spectrum, and thus a broad distinction is exhibited
between the action of the sun's rays on vegetable juices and on
argentine compounds, the latter being most sensibly affected by the
invisible rays beyond the violet.

It may also be observed, that the rays effective in destroying a given
tint, are in a great many cases, those whose union produces a color
complementary to the tint destroyed, or, at least, one belonging to
that class of colors to which such complementary tint may be preferred.
For instance, yellows tending towards orange are destroyed with more
energy by the blue rays; blues by the red, orange and yellow rays;
purples and pinks by yellow and green rays.

V. AMPHITYPE.

This process is a discovery of Sir John Herschel and receives its name
from the fact that both negative and positive photographs can be
produced by one process. The positive pictures obtained by it have a
perfect resemblance to impressions of engravings with common printer's
ink. The process, although not yet fully carried out, promises to be
of vast utility.

Paper proper for producing an amphitype picture may be prepared either
with the ferro-tartrate or the ferro-citrate of the protoxide, or the
peroxide of mercury, or of the protoxide of lead, by using creams of
these salts, or by successive applications of the nitrates of the
respective oxides, singly or in mixture, to the paper, alternating with
solutions of the ammonia-tartrate or the ammonia-citrate of iron, the
latter solution being last applied, and in more or less excess. I
purposely avoid stating proportions, as I have not yet been able to fix
upon any which certainly succeed. Paper so prepared and dried takes a
negative picture, in a time varying from half an hour to five or six
hours, according to the intensity of the light; and the impression
produced varies in apparent force from a faint and hardly perceptible
picture to one of the highest conceivable fulness and richness both of
tint and detail, the color being in this case a superb velvety brown.
This extreme richness of effect is not produced unless lead be present,
either in the ingredients used, or in the paper itself. It is not, as
I originally supposed, due to the presence of free tartaric acid. The
pictures in this state are not permanent. They fade in the dark,
though with very different degrees of rapidity, some (especially if
free tartaric or citric acid be present) in a few days,