ELECTRIC LIGHTS. 169 



organizes them into a symmetrical arrangement, with 

 plane sides at their junctions, instead of curved sides, 

 as single ones have. 



Pour a few drops of soap solution upon a piece of 

 window-glass, and with the finger spread it over a sur- 

 face four or five inches square. Then with a common 

 blow-pipe or glass dropping-tube blow bubbles upon the 

 glass, starting them at any point. They had better not 

 be blown more than an inch or two in diameter. If a 

 second bubble be started at a point an inch or two from 

 the first one, the two will rush together ; a third one 

 will join to the two so that the interior angles at their 

 junction will equal 120. As others are added all will 

 change their surfaces of adhesion and their relative 

 positions. 



By placing the glass upon the vertical projector (pp. 

 42 and 43), the growth, motions, and symmetrical ar- 

 rangement may be seen and studied by a hallful at once. 



VIBRATION OF FILMS. 



If the end of a tube like a glass lamp-chimney be 

 dipped into a soap solution, a film will remain over the 

 end when it is taken out of it. If now a beam of par- 

 allel rays of light be directed upon this film, some of the 

 light will be reflected from it. Place a lens four 

 or five inches in diameter and ten or twelve inches 

 focus so as to project this reflected light upon the screen 

 or white wall. An enlarged image of the film will be 

 seen upon which a series of spectral colors will appear. 

 If a sound be made by the voice at the open end of 

 the tube, the film will be thrown into vibrations similar 

 in form to the air waves that produce them. These 

 vibratory movements will show upon the screen as 

 a curious network which will change for each different 



