the horizontal surface is seen, and then to look as before at the glass
instead of through it. Having held the glass carefully to see the
color it appears to be when in front of each of the two surfaces, ask the
student to move the glass up and down, slowly at first and then as
rapidly as possible, so that it shall transmit first the color of the ver-
tical surface and then that of the horizontal. If tkis motion is con-
tinued and the student looks at the glass, he will discover that the
difference in color between the two surfaces is much greater than what
he thought he saw and what his drawing represents ; and he will at
the same time have gained a true impression of values and learned
to use his glass properly. The use of paper or cloth of one color is
of great assistance in enabling the pupil to look at the glass instead
of through it, and this experiment should be the first lesson of pupils
old enough to apply it, and should be repeated as often as is neces-
sary to enable all the students to use the glass properly ; for if they
do not use it properly they injure their eyes and destroy their best
chance of becoming independent and able to correct their own light
and shade studies. When large masses are compared, the blur glass
should always be held as far as possible from the eye ; and when any
part is studied it should be held, if possible, far enough from the eye
to cause the part to blur into a formless mass of color which fills the
entire glass, and which is to be looked at as if upon the glass.
