176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



gens, on the one hand, and the older and more widely favored cor- 

 puscular theory, on the other. Finally Newton definitely adopted the 

 corpuscular theory of light, but still thought that heat radiation 

 might be due to a wave motion. Newton was beyond question one 

 of the greatest and most progressive thinkers of all time and, had 

 he been alive, would have been among the first to recognize the con- 

 vincing evidence of Young's experiment, but Newton dead proved 

 a most formidable obstacle to progress. 



Young not only proved the wave theory but actually measured 

 the wave lengths of different colored light and showed a progres- 

 sive increase in wave length as we go from violet through blue, 

 green, and yellow to the extreme red. Thus the wave length of 

 yellow light situated about midway in the orderly spaced band of 

 colors which we call the normal spectrum is approximately 0.00059 

 mm. or one fifty-thousandth of an inch. From this time on to 

 the middle of the nineteenth century, increasing and more convinc- 

 ing evidence of the wave theory slowly accumulated until Foucault 

 actually measured the velocity of light in water and found it less 

 than in air by an amount in exact accordance with the requirements 

 of the wave theory, while the corpuscular theory required just the 

 reverse; i. e., a greater velocity in water than in air. Thus a wave 

 disturbance theory was finally established. 



Fifteen years later Maxwell proposed a new theory of the nature 

 of light waves which he based on Faraday's experiments. Maxwell's 

 theory at the time it was proposed proved as abstruse and baffling 

 for physicists of his day as we find Einstein's generalized theory of 

 relativity in ours. In fact, Maxwell was the Einstein of his gen- 

 eration, and he succeeded in puzzling everybody. Maxwell's theory 

 asserts that light is an electric wave disturbance in the ether caused 

 by astoundingly rapid oscillations of minute electric charges in the 

 source of light. But nobody then knew anything about rapidly oscil- 

 lating electric charges in light sources, and another outstanding 

 difficulty in believing that light waves were electric waves was that 

 nobody had discovered any electric waves with which to compare 

 them, nor was there any clearly recognized indication that electric 

 waves were possible. 



In the following decade the Berlin Academy of Sciences became 

 interested in Maxwell's theory and offered a prize to anyone who 

 would give experimental proof of its fundamental assumption. Not 

 long after this prize was offered, von Helmholtz called the attention 

 of one of his most promising students to it. That student was Hein- 

 rich Hertz, who later not only obtained electric waves some 2 feet 

 long in the laboratory but went further and performed a number of 

 experiments demonstrating that these waves, like light, were reflected 



