circular or, rather, spherical waves in every direction from the starting-point. It is only waves of light which are reflected back from two points very, very near each other, which produce the colors of interference. Circles which do not have the same center cut each other only at two points; but, the nearer the two centers are, the more nearly the circumferences coincide. When the light comes back colored from a piece of mother-of-pearl, it is because the waves are reflected back from lines so close together that you can not see them, except under a very high power of the microscope, and so they interfere. Metal may be ruled with lines that give back the same sort of color, and perfect impressions in black sealing-wax of the colored pearl will show colors in the same way.
The colors which flit over the surface of a soap-bubble each tells the story of the thickness of the film at that point. These films are exposed to the movement and drying effects of the air, and to the irregular puffs of air entering from the mouth in blowing them; but if a film can be secured from these influences and allowed to become gradually and evenly thinner, even and regular colors appear. Blow a soap-bubble in a watch-glass filled with the soapy fluid. Let it sit in a saucer in which there is also some of the fluid, and cover with a clear glass tumbler the instant the bubble a little overhangs the watch-glass. The soapy fluid in the saucer prevents the air from getting in or out of the tumbler. Such a bubble blown from soap-suds made of distilled water and white Castile soap, which had been standing a very long while and become crystal clear, lasted for three hours and a quarter. It had no colors upon it when covered. They began to form at once: broad bands of pink and green slipped down from the apex; then came closer and more vivid rings of color; at last a black spot appeared, which grew in size. In the long-lived bubble just spoken of, the whole upper part became a metallic gray, covered with clouds of darkness and velvety black spots, the colors being crowded from the apex down to the edges. That these appearances are all due to interference is proved by the fact that, when the light by special means is prevented from reflection at one of the surfaces of the film, the color disappears.
There is no special advantage for home experiments in having a bubble last so long. Very much the same changes occur in a bubble which lasts for half an hour as in one that lasts for three hours, only they occur more quickly.
The colors of films are rarely, if ever, pure prismatic colors; they are the resultant of certain colors left after the extinction of others. Various shades of green, from almost gold to the intensest emerald green, orange dusky with red, red magenta-colored from the admixture of blue waves, and so on, are the colors seen.
