THE LAWS OF FILMS. 631 



and others by simply carefully singing into the rubber tube. The 

 colors change constantly, and are so rich and gorgeous that they 

 seem to have lost their transparency and to be metallic plates, ex- 

 cept for their streaming, swirling colors. The diaphragms should, 

 as was said before, be blackened, and the film seen projected 

 against a black background. I simply use a piece of black mate- 

 rial placed behind and a little below the bell-glass on which the 

 diaphragm rests. 



These interference colors do not require a film of any special 

 substance, or, indeed, of any substance at all. The air between 

 two plates of clean, clear glass, pressed together and worked with 

 the fingers till they are as close as possible, gives beautiful rings 

 and fringes of color. A crack in the center of a block of clear 

 ice, where there is not even air, but only empty space, or, rather, 

 the ether that fills all space, gives out gorgeous colors of inter- 

 ference. 



The colors of iridescent glass are due to interference. In its 

 manufacture, by some chemical means, the surface film has been 

 made different from the glass below, and so acts as the soap film 

 does, and gives out its lovely tints. A drop of turpentine on the 

 surface of water on a black tray shows fringes of color from the 

 same cause. 



One of the most beautiful examples of interference color may 

 be seen at the Metropolitan Museum of Art in New York, in the 

 Cesnola collection of ancient glass. Originally this glass was 

 evidently ordinary transparent glass. From some cause the sur- 

 face has been acted upon till it lies in thin films one upon the 

 other, sending back to the eye the most gorgeous interference 

 colors. By the courtesy of Dr. Isaac Hall, the curator of the mu- 

 seum, I was enabled to examine some fragments of this glass mi- 

 croscopically. 



The whole surface is made up of a series of films of the most 

 exquisite delicacy. There are tiny cavities united by a network 

 of lines from which the decomposition has spread laterally in 

 every direction. Flakes come off with the lightest touch, so thin 

 that it seems impossible they should be capable of subdivision, 

 and yet a good two-thirds glass (about one hundred diameters) 

 shows it to be made up of a number of superposed plates. The 

 fact that the color of this glass is due to interference is proved by 

 putting a drop of alcohol or oil upon a flake, when the colors dis- 

 appear or are entirely changed. As the liquid dries, the colors 

 gradually come back. 



The beauty of this glass under the microscope is simply inde- 

 scribable. Gold and silver, exquisitely wrought, and vivid with 

 every known jewel, would be tame and colorless beside it (Fig. 

 10, A). The films, as they come off, are in many cases not ordi- 



