Another simple and very interesting experiment is to twist a copper wire into the shape of a tennis-racket or battledore, dip it into the fluid and set it upright under the tumbler. If the saucer is partially filled with yellow beeswax, melted and allowed to harden, this can be very easily done. The colors in this case come down in bars, in the same order as they did on the bubble; the black spot is much larger and more irregular in shape. In one instance, with a simple soap solution, this spot of intense black covered three quarters of the frame before the breaking of the film. Many films may break before one is secured which will last so as to show these effects.
The cause of these regular rings and bars of color is that the film gradually thins from the top, by the slow streaming off or evaporation of the suds from the film, and for each definite thickness a definite color appears. The black spot which comes last of all shows that the film at that place is just one half a wave-length of light in thickness, a size entirely too small for our conception, though it can be told in numbers. The length of a wave of red light is about 137000 of an inch, and of all the other colors smaller.
The circulation and changes in the film are most curiously revealed by the movement of flecks of color on its surface.
There are other ways of making inequalities in the film, which are revealed by the colors. A little instrument, called the phoneidoscope,
Fig. 8.—Phoneidoscope.
A, bell-glass; B, elbow; C, India-rubber tube; D, wire support; F, upper half of mouth-piece; E, lower half of mouth-piece; G, diaphragm.
which may be either bought or very easily made, shows most beautiful figures which start into shape in answer to musical notes sung or words spoken into it. It is in all its forms a modification of, or improvement upon, this idea: an inch tube of India rubber of any length, with a funnel on one end and a mouth-piece on the other, diaphragms of thin metal or varnished cardboard being placed across the mouth of the funnel with holes of various
