A soap solution is made up as follows: One part of fresh Castile soap is dissolved in forty parts of warm water; when cool, three parts of the solution are mixed with two parts 
FIG. 15of glycerine. The mixture is cooled to a temperature of 3° or 4° C., and filtered. A soap film is formed by dipping into the solution a short piece of wide glass tubing. Removing the tube and placing it so that the film is vertical, a series of beautifully colored bands appear, the colors being deeper at the top and gradually fading into barely perceptible alternations of pink and green near the bottom. The bands broaden out as the film gets thinner, but the succession of colors remains the same and may be described as follows: The top of the film is black; then the colors in the first band are bluish gray, white, yellow, and red; those in the second band are, in order, violet, blue, green, yellow, red; the third band is blue, green, yellow, and red; and the succeeding bands green and red. The colors are best observed by using the film as a mirror to reflect the light from a white wall; or the light from a lantern may be reflected to a lens which forms an image of the film on a screen.
The colors of thin films and of interference phenomena
