e it is produced by a section of
the cone L a l L, of which the external rays L a L l, are red; if the
screen be moved to the other side of o, the luminous circle will be
bordered with violet, because it will be a section of the cone M a M l,
of which the exterior rays are violet. To avoid the influence of
spherical aberration, and to render the phenomena of coloration more
evident, let an opaque disc be placed over the central portion of the
lens, so as to allow the rays only to pass which are at the edge of the
glass; a violet image of the sun will then be seen at v, red at r, and,
finally, images of all the colors of the spectrum in the intermediate
space; consequently, the general image will not only be confused, but
clothed with prismatic colors."

To overcome the difficulty arising from the chromatic aberration, the
optician has only to employ a combination of lenses of opposite focal
length, and cut from glass possessing different refrangible powers, so
that the rays of light passing through the one are strongly refracted,
and in the other are bent asunder again, reproducing white light.

To the photographer one of the most important features, requiring his
particular attention, is, that he be provided with a good lens. By the
remarks given in the preceding pages, he will be enabled, in a measure,
to judge of some of the difficulties to which he is occasionally
subjected. We have in this country but two or three individuals who
are giving their attention to the manufacture of lenses, and their
construction is such, that they are quite free from the spherical or
chromatic aberration.

CHAPTER V.

To make Plates for the Daguerreotype--Determining the Time of Exposure
in the Camera--Instantaneous Process for Producing
Daguerreotype--Galvanizing the Daguerreotype Plate--Silvering
Solution--Daguerreotype without Mercury--Management of Chemicals--Hints
and Cautions--Electrotyping--Crayon Daguerreotypes--Illuminated
Daguerreotypes--Natural Colors in Heliography--Multiplying
Daguerreotypes on one Plate--Deposit in Gilding--Practical Hints on the
Daguerreotype.

TO MAKE PLATES FOR THE DAGUERREOTYPE.

I do not give the method employed by our regular plate manufacturers;
this is not important, as the operator could not possibly profit by it
from the fact of the great expense of manufacturing. The following
will be found practical:

Procure a well planished copper plate of the required size, and well
poli