Often it is desired to produce certain effects in lighting which demand a different-colored light than that given by the modern tungsten lamp. As an example, a soft, mellow light is sometimes desired similar to that given by the old carbon-filament lamp. In such cases it is a great mistake to install the carbon lamps on account of their exceedingly poor efficiency.

The ordinary tungsten lamp has an average efficiency of about 1.2 watts per candlepower, while the carbon-filament lamp requires about 3 watts per candlepower. Therefore, it is much more economical to color the globe of the tungsten lamp so as to produce the required color than to use the carbon lamp. Of course, both lamps must be colored when any color other than a soft, yellow light is desired.

A cheap coloring solution may be made as follows: Soak a small amount of gelatin in cold water for several hours, then boil it and strain it through a piece of fine cloth. While the solution is still hot, add a small quantity of aniline dye of the desired color that has been previously mixed in a small quantity of cold water. The lamps are dipped in the solution and then allowed to cool in a vertical position so that the coating will be more uniform.

A more satisfactory coloring solution may be made from celluloid. Obtain quite a number of old photographic films and remove all the gelatin by washing them in hot water. Then dissolve them in a solution of equal parts of ether and alcohol. Add the coloring solution and dip the lamps. The coating produced by this method is impervious to water.