is all you need, but it will be easily broken
if unprotected.

Do not try to look _through_ the glass at your subject, but _at_ the
glass and the image on it.

=The Claude Loraine Mirror.=--This is a curved mirror with a black
reflecting surface. The object is reflected on it, _reduced_ both in
size and pitch. It concentrates the masses and the color, and so helps
to distinguish the relative values.

You can make a mirror of this sort for yourself by painting the back
of a piece of plate glass black. The real Claude Loraine mirror is
expensive.

=The Common Mirror= is also very helpful in distinguishing values. It
reduces the size of things, and reverses the drawing so that you see
your subject under different conditions, and a fresh eye is the
result. Place the group and your painting side by side, if you are
painting still life, and look at both at the same time in the mirror.
Do the same with a portrait and the sitter.

=Diminishing Glass.=--Much the same effect can be had by using a
double concave lens. The picture is not reversed, but it is reduced,
and the details eliminated.

In using any of these means you must remember that it is always the
relations and not the things you are studying; and the most useful of
these aids is the blur glass, because you cannot possibly see anything
in it but the values and color masses, everything else being blurred.

CHAPTER XVIII

PERSPECTIVE

There are two kinds of perspective, linear and aerial. The former has
to do with the manner in which horizontal lines appear to converge as
they recede from the foreground, and so produce the effect of
distance. The latter has to do with the effect of distance, which is
due to the successive gradations of gray in color noticeable in
objects farther and farther away from the observer.

=Aerial Perspective.=--To the student, aerial is _color_ perspective,
because of the modifications which colors undergo when removed to a
distance. Modifications of tone are largely due to varying distance,
and so aerial perspective is largely a matter of _values_. That they
are due to the greater or less thickness of the atmosphere is only a
matter of interest, not of importance, to the artist; the important
thing to him is that the careful study of values is necessary to
relief, perspective, and particularly, atmosphere and envelopment in a
picture.

To the student, aerial persp