ed
spherical aberration, and to it is due the want of distinctness which
is frequently noticed around the edges of pictures taken in the camera.
To secure a camera with a flat, sharp, field, should be the object of
every operator; and, in a measure, this constitutes the great
difference in cameras manufactured in this country.

Spherical aberration is overcome by proper care in the formation of the
lens: "It can be shown upon mathematical data that a lens similar to
that given in the following diagram--one surface of which is a section
of an ellipse, and the other of a circle struck from the furthest of
the two foci of that ellipse--produces no aberration.

"At the earliest period of the employment of the camera obscura, a
double-convex lens was used to produce the image; but this form was
soon abandoned, on account of the spherical aberration so caused.
Lenses for the photographic camera are now always ground of a
concavo-convex form, or meniscus, which corresponds more nearly to the
accompanying diagram."

[Illustration: Fig. 11 (amdg_11.gif)]

Chromatic Aberration is another difficulty that opticians have to
contend with in the manufacturing of lenses. It will be remembered,
that in a former page (133) a beam of light is decomposed by passing
through a glass prism giving seven distinct colors--red, orange,
yellow, green, blue, indigo and violet.

Now, as has been said before, the dissimilar rays having an unequal
degree of refrangibility, it will be impossible to obtain a focus by
the light passing through a double-convex lens without its being
fringed with color. Its effect will be readily understood by reference
to the accompanying cut.

[Illustration: Fig. 8b (amdg_8b.gif)]

If L L be a double convex-lens, and R R R parallel rays of white light,
composed of the seven colored rays, each having a different index of
refraction, they cannot be refracted to one and the same point; the red
rays, being the least refrangible, will be bent to r, and the violet
rays, being the most refrangible, to v: the distance v r constitutes
the chromatic aberration, and the circle, of which the diameter is a l,
the place or point of mean refraction, and is called the circle of
least aberration. If the rays of the sun are refracted by means of a
lens, and the image received on a screen placed between C and o, so as
to cut the cone L a l L, a luminous circle will be formed on the paper,
only surrounded by a red border, becaus