THE PHOTOGRAPHY OF COLORS. 543 



may result from the superposition of several simple vibrating 

 motions. In general, the coloring of bodies results from the 

 diffusion of the light-rays which illuminate them. The bodies 

 absorb a part of the rays and reflect others. The mingling of the 

 reflected rays produces on the eye the impression of a definite 

 tint. A cloth appears red to us because it reflects chiefly the red 

 light and absorbs all the other colors. If it reflects all the solar 

 rays as they are, it appears white to us ; if, instead of reflecting 

 them, it absorbs them, it appears black. 



The origin of colors, therefore, we see, depends upon a phys- 

 ical or mechanical and not on a chemical cause. The white light 

 which comprises them all is only the resultant of the infinity of 

 the colors that exist and succeed one another in gradation from 

 the red to the violet. This may be easily perceived by letting a 

 ray of sunlight pass through a crystal cut in facets. 



To comprehend fully the direction of M. Lippmann's thoughts 

 before hitting on the photography of colors by the application 

 of the theory of vibratory motions, we must say a little more con- 

 cerning the phenomena of interference. When two sound-waves 

 meet, there occurs, according to certain specific conditions, either 

 an amplification of the sound by their combination or a destruc- 

 tion of it by their collision. The principle of the interference of 

 sound was demonstrated by Colonel Napoleon Savart in 1839, by 

 an experiment which is not so well known as it should be. This 

 sagacious officer placed in front of the principal wall of the citadel 

 in which he was garrisoned a bell which he rung by striking it 

 with a hammer. The bell thus became the center of a direct wave 

 which was propagated to the wall of the citadel and reflected 

 from it. In other words, the action of the sound was brought to 

 bear upon the wall, which sent it back to the starting point and 

 thus could give rise to the phenomenon of interference. Some 

 among the soldiers stationed along the line between the bell and 

 the wall observed a distinct re-enforcement of the sound ; while 

 others, placed exactly at the points of interference, heard nothing. 



What -passed in Colonel Savart's experiments is reproduced in 

 the same manner with light- vibrations. Just as sound added to 

 sound may produce either silence or amplification of the sound, 

 so light added to light may produce darkness or amplification of 

 the luminous effect. When direct light falls upon a mirror, it 

 meets on the way the light that was previously reflected, and 

 wherever the vibrations agree in direction the brightness is in- 

 creased, whereas it is extinguished wherever they are opposed 

 to one another. The space in front of the mirror will therefore 

 be divided into successive sections or stratifications. In some, 

 the light will be of its highest brightness ; in others, on the other 

 hand, there will be complete darkness. It can easily be deter- 



