628 THE POPULAR SCIENCE MONTHLY. 



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 cen- 

 ter 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 impres- 

 sions 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 ap- 

 peared, 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 appear- 

 ances 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. 



