THE LAWS OF FILMS. 



629 



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 thick- 

 ness 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 3 n \ Q & of an inch, and of all the other colors smaller. 



The circulation and changes in the film are most curiously re- 

 vealed 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 phonei- 



FlG. 81 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. 



doscope, 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 modifi- 

 cation 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 



