540 THE POPULAR SCIENCE MONTHLY. 



Becquerel process lost most of its value by its instability. The 

 science and experimental skill of the celebrated physicist could 

 not overcome this obstacle, on which all who tried to accomplish 

 photochromy by the method of direct impression were successively 

 wrecked. 



The chemists Niepce de Saint- Victor, 1851 to 1866, Testud de 

 Beauregard, in 1855, and Poitevin, in 1865, tried to secure the colors 

 by means of chemical substances, but were never able to fix their 

 proofs, or to keep them perfect in the presence of light. After the 

 chemists came the photographers ; after the photographers, the 

 men with empirical methods. Then came incomplete geniuses, like 

 Charles Cros, reproducing the colors by superposed prints, with- 

 out using a direct method, or any effective one. Yet Cros was 

 wonderfully endowed with inventive genius. He had notions 

 about everything. He was one of the first persons, if not the 

 first, to dream of phonography. He occupied himself with the 

 transmission of images to a distance. Occasionally he satisfied 

 himself also with inventing things of a simpler and more positive 

 character, such as his famous paste, a little microscopic box of 

 which would afford ink enough for a whole lyceum for an entire 

 year. 



What no one could obtain by any chemical method, M. Lipp- 

 mann has realized from the theory of vibratory motions. In the 

 soap bubbles, with which every one is familiar, colors of rare 

 brilliancy detach themselves from the thickness of the liquid 

 films, which are themselves colorless. Whenever a transparent 

 body is drawn out into a very thin film it appears with iridescent 

 hues, although it may be made of a colorless substance. The 

 coloration arises from the fact that the light reflected from the 

 two faces of the film has not passed over the same distance. In 

 other words, the light plays by its reflection upon the two planes 

 that bound the film. The result is that the light-rays cross each 

 other and give rise to a phenomenon which is called interference. 

 On closely examining the brilliant tints of the soap bubbles we 

 easily recognize the different colors of the spectrum. 



Newton first discovered the causes of coloration, and, to render 

 them more tangible, he devised the experiment of " Newton's 

 colored rings." On an absolutely plane glass he fixed, by its 

 spherical face and without fastening it in any other way, a con- 

 vex lens ; the lens, consequently, did not touch the glass except 

 at one point, all the other points remaining separated from it by 

 sections of air which grew thicker as they were farther removed 

 from the point of contact. When this apparatus is illuminated 

 by a monochromatic light, such as the yellow light given by a 

 lamp burning salted alcohol, there is at once remarked a central 

 black spot on the glass, surrounded by concentric rings alter- 



