less than the other, and the divergence has
increased, throwing, the focus further from the lens."

[Illustration: Fig. 9 (amdg_9.gif)]

These remarks have been introduced here as being important for those
who may not understand the principles of enlarging or reducing pictures
in copying.

I would remark that the points F and A, in Fig. 9, are termed
"conjugate foci."

If we hold a double-convex lens opposite any object, we find that an
inverted image of that object will be formed on a paper held behind it.
To illustrate this more clearly, I will refer to the following woodcut:

[Illustration: Fig. 10 (amdg_10.gif)]

"If A B C is an object placed before a convex lens, L L, every point of
it will send forth rays in all directions; but, for the sake of
simplicity, suppose only three points to give out rays, one at the top,
one at the middle, and one at the bottom; the whole of the rays then
that proceed from the point A, and fall on the lens L L, will be
refracted and form an image somewhere on the line A G E, which is drawn
direct through the centre of the lens; consequently the focus E,
produced by the convergence of the rays proceding from A, must form an
image of A, only in a different relative position; the middle point of
C being in a direct line with the axis of the lens, will have its image
formed on the axis F, and the rays proceeding from the point B will
form an image at D; so that by imagining luminous objects to be made up
of all infinite number of radiating points and the rays from each
individual point, although falling on the whole surface of the lens, to
converge again and form a focus or representation of that point from
which the rays first emerged, it will be very easy to comprehend how
images are formed, and the cause of those images being reversed.

"It must also be evident, that in the two triangles A G B and D G E,
that E D, the length of the image, must be to A B, the length of the
object, as G D, the distance of the image, is to G B, the distance of
the object from the lens.

It will be observed that in the last cut the image produced by the lens
is curved. Now, it would be impossible to produce a well-defined image
from the centre to the edge upon a plain surface; the outer edges would
be misty, indistinct, or crayon-like. The centre of the image might be
represented clear and sharp on the ground glass, yet this would be far
from the case in regard to the outer portions. This is call