The distinguished French chemist, M. Chevreul, who has devoted so much
attention to the subject of color, has lately published a memoir on
painted windows, in which there are many points which deserve the
attention of artists and others who are interested in the manufacture
of colored glass. It has often been much noticed that old stained glass
windows have a much richer effect than modern ones, and M. Chevreul,
speaking of this superiority, attributes it to what moderns regard as
defects. In the first place, much of the ancient glass is of unequal
thickness, and so presents convex and concave parts, which refract the
light differently and produce an agreeable effect. In the next place
the old colored glass is not a colorless glass, to which has been added
the particular coloring material, such as protoxide of cobalt, &c. Old
glass contains a good deal of oxide of iron, which colors it green, and
to this must be attributed the peculiar effects of antique glass,
colored by cobalt and manganese. M. Chevreul appears to think that
modern stained glass is too transparent to produce the best effects. M.
Regnault, the chemist, has recommended that all this kind of stained
glass should be cast, to avoid the monotonous effect of plain surfaces
on the light; and also that foreign substances should be mixed with the
glass to diminish its transparency.
Many attempts have been made to color with ruby or other colors gas
shades, so as to throw on surrounding objects the color of the glass;
but in no case has the ray of light passing through colored glass, to
refract the shade, been successful.
But when a ray of solar light is passed through a colorless prism, it
is refracted, and forms, when thrown on a wall or screen, a broad band
of colored light,--red, orange, yellow, green, blue, indigo, and
violet,--which is known as the prismatic or solar spectrum.