on of the glass not the slightest wavy
appearance, or dark spot should be detected; and a curvature which as
much as possible prevents spherical aberration should be secured. The
effect produced by this last defect is a convergence of perpendiculars,
as for instance; two towers of any building, would be represented as
leaning towards each other; and in a portrait the features would seem
contracted, distorted and mingled together, so as to throw the picture
out of drawing and make it look more like a caricature than a likeness.
If the lens be not achromatic, a chromatic aberration takes place,
which produces an indistinct, hazy appearance around the edges of the
picture, arising from the blending of the rays.

The diameter and focal length of a lens must depend in a great measure
on the distance of the object, and also on the superficies of the plate
or paper to be covered. For portraits one of 1 1/2 inches diameter,
and from 4 1/2 to 5 1/2 inches focus may be used; but for distant
views, one from 2 inches to 3 inches diameter, and from 8 to 12 inches
focal length will answer much better. For single lenses, the aperture
in front should be placed at a distance from it, corresponding to the
diameter, and of a size not more than one third of the same. A variety
of movable diaphragms or caps, to cover the aperture in front, are very
useful, as the intensity of the light may be modified by them and more
or less distinctness and clearness of delineation obtained. These caps
alway come with Voitlander instruments and should be secured by the
purchaser.

Though the single acromatic lens answers very well for copying
engravings; taking views from nature or art, for portraits the double
should always be used. The extensive manufacture of the most approved
cameras, both in Europe and in this country, obviates all necessity for
any one attempting to construct one for their own use. Lenses are now
made so perfect by some artisans that, what is called the "quick
working camera" will take a picture in one second, while the ordinary
cameras require from eight to sixty.

The camera in most general use is that manufactured by Voitlander and
Son of Germany. Their small size consists of two seperate acromatic
lenses; the first, or external one, has a free aperture of 1 1/2
inches; the second, or internal, 1 5/8 inches; and both have the same
focus, viz: 5 3/4 inches. The larger size differs from the smaller.
The inner lens is a