o secure a
well-delineated representation of any object we choose to picture.

When a ray of light passes from one medium to another, and through that
into the first again, if the two refractions be equal, and in opposite
directions, no sensible effect will be produced.

The reader may readily comprehend the phenomena of refraction, by means
of light passing through lenses of different curves, by reference to
the following diagrams:--

[Illustration: Fig. 5, 6, 7 (amdg_5.gif)]

Fig 5 represents a double-convex lens, Fig. 6 a double-concave, and
Fig. 7 a concavo-convex or meniscus. By these it is seen that a
double-convex lens tends to condense the rays of light to a focus, a
double-concave to scatter them, and a concavo-convex combines both
powers.

If parallel rays of light fall upon a double-convex lens, D D, Fig. 8,
they will be refracted (excepting such as pass directly through the
centre) to a point termed the principal focus.

[Illustration: Fig. 8a (amdg_8a.gif)]

The lines A B C represent parallel rays which pass through the lens, D
D, and meet at F; this point being the principal focus, its distance
from the lens is called the focal length. Those rays of light which
are traversing a parallel course, when they enter the lens are brought
to a focus nearer the lens than others. Hence the difficulty the
operator sometimes experiences by not being able to "obtain a focus,"
when he wishes to secure a picture of some very distant objects; he
does not get his ground glass near enough to the lenses. Again, the
rays from an object near by may be termed diverging rays. This will be
better comprehended by reference to Fig. 9, where it will be seen that
the dotted lines, representing parallel rays, meet nearer the lenses
than those from the point A. The closer the object is to the lenses,
the greater will be the divergence. This rule is applicable to
copying. Did we wish to copy a 1/6 size Daguerreotype on a 1/16 size
plate, we should place it in such a position to the lenses at A that
the focus would be at F, where the image would be represented at about
the proper size. Now, if we should wish to copy the 1/6 size picture,
and produce another of exactly the same dimensions, we have only to
bring it nearer to the lenses, so that the lens D E shall be
equi-distant from the picture and the focus, i. e. from A to B. The
reason of this is, that the distance of the picture from the lens, in
the last copy, is