coloring elements in the
same glass the "subtractive" method of color-mixture is utilized. For
example, if a green glass is desired, yellowish green chromium glass may
be used as a basis. By the addition of some blue-green due to copper,
the yellow rays may be further subdued so that the resulting color is
green.

The primary colors for this method of color-mixture are the same as
those of the painter in mixing pigments--namely, purple, yellow, and
blue-green. Various colors may be obtained by superposing or intimately
mixing the colors. The resulting transmission (reflection in the case of
reflecting media such as pigments) are those colors commonly transmitted
by all the components of a mixture. Thus,

Purple and yellow = red
Yellow and blue-green = green
Blue-green and purple = blue

The colors produced by adding lights are based not on the "subtractive"
method but on the actual addition of colors. These primaries are red,
green, and blue and it will be noted that they are the complementaries
of the "subtractive" primaries. By the use of red, green, and blue
lights in various proportions, all colors may be obtained in varying
degrees of purity. The chief mixtures of two of the "additive" primaries
produce the "subtractive" primaries. Thus,

Red and blue = purple
Red and green = yellow
Green and blue = blue-green

Although the coloring media which are permanent under the action of
light, heat, and moisture are relatively few, by a knowledge of their
spectral characteristics and other principles of color the expert is
able to produce many permanent colors for lighting effects. The additive
and subtractive methods are chiefly involved, but there is another
method which is an "averaging" additive one. For example, if a warm tint
of yellow is desired and only a dense yellow glass is available, the
yellow glass may be cut into small pieces and arranged upon a colorless
glass in checker-board fashion. Thus a great deal of uncolored light
which is transmitted by the filter is slightly tinted by the yellow
light passing through the pieces of yellow glass. If this light is
properly mixed by a diffusing glass the effect is satisfactory. These
are the principal means of obtaining colored light by means of filters
and by mixing colored lights. By using these in conjunction with the
array of light-sources available it is possible to meet most of the
growing demands. Of course, the id