Nipher — Physics During the Last Century. 113 



record the statement that his ** chief objection to the undu- 

 latory theory of light was that he could not think the Creator 

 guilty of so clumsy a contrivance as the filling of space with 

 ether in order to produce light." 



In those days they tried to settle such questions by attor- 

 neys who argued, and ridiculed, and quoted authorities and 

 precedents. Lord Brougham, who was a prominent figure 

 of that day, made the most ludicrous efforts of this 

 kind. He assailed Thomas Young, the great exponent 

 of the wave theory, with the most bitter personalities. Lord 

 Brougham's abilities and opportunities did not justify any 

 well-grounded hope that he could know anything about a 

 theory which must be tested by mathematical analysis and del- 

 icate experiment, but his powers of ridicule and invective were 

 of a high order. For a time he prevailed with the British 

 public as against Thomas Young. It was in 1801 that Young 

 showed that Newton's rings and the colors of thin plates 

 might be explained by the wave theory. Ten years later 

 Fresnel gave the subject an elaborate mathematical discussion, 

 and designed the most searching experimental tests, in which 

 wave length was determined by interference phenomena. 

 By the wave theory, it was easy to explain how the super- 

 position of two luminous pencils might produce darkness. 

 The advocates of Newton's ideas yielded very slowly, but the 

 measurement of the velocity of light in various media gave 

 the final evidence which could no longer be questioned. 

 Newton's theory required that the velocity of light should be 

 greater in matter than in a vacuum, and the reverse was found 

 to be the case, as the wave theory demanded. The velocity 

 of light was measured over terrestrial distances about the 

 middle of the century. In 1850 Foucault measured the time 

 required by light to travel over a distance of about 20 meters. 

 This time is about ttotoVoo-o second, an interval that bears 

 about the same relation to the second that the second does to 

 six months. And this minute interval of time is to be meas- 

 ured with precision. 



This measurement was made possible by a method used by 

 Wheatstone in determining the duration of an electric spark. 

 A beam of light is reflected from a rapidly revolving mirror, 



