OF ARTS AND SCIENCES. 387 



theory of this method, the velocity of light was placed at 187,221 

 miles. This value exceeded those obtained by Cornu or Michelson ; 

 but this might be explained by the color of the light used in the differ- 

 ent experiments. Mr. Young and Professor Forbes made some ex- 

 periments with lights of different colors, iu confirmation of this view. 

 But Professor Michelson compared his 318 observations with sunlight 

 and 267 observations with electric light, and found that the difference 

 was in the opposite direction ; and in a differential experiment, when 

 half the slit was covered with red glass, he found no displacement. 

 Young and Forbes were attracted to their experiments on the velocity 

 of light by Maxwell's speculations on the electromagnetic theory of 

 light, and also as promising the most accurate method of obtaining the 

 parallax and distance of the sun. Their velocity of light combined 

 with Struve's constant of aberration made the sun's parallax 20".445, 

 and its distance 93,223,000 miles. 



When Arago, in 1838, suggested to the French Academy an exper- 

 iment on the velocity of light, and explained his method of making it, 

 which was essentially the one afterwards adopted by Foucault, he had 

 in view the settlement of the long controversy between the advocates 

 of the corpuscular and undulatory theories. Almost all of the differ- 

 ent classes of phenomena in geometrical optics can be explained by 

 either one of these theories, though even here the undulatory has the 

 advantage of greater simplicity. But in one respect the two theories 

 are antagonistic. According to the corpuscular theory, light should 

 move faster in glass or water than in air, for example. The undula- 

 tory theory reversed this proposition. Here was an experimentum 

 cruets. In 1850, Fizeau and Foucault made the experiment, each in 

 his own way, and in both experiments the result was in favor of the 

 theory of undulations. It has been shown that in the case of air 

 alone lengths of many thousand feet are practicable. But the absorb- 

 ing power of water prevents the use of greater lengths than about 

 10 feet. Light would pass through 10 feet of air in less time than 

 T^^o ^^ ^ second ; and the difference of time for air and water would 

 be only a fraction of that small fraction. Hence the exceeding deli- 

 cacy of the experiment. 



In 1883, Mr. Michelson, at the request of Professor Newcomb, re- 

 peated Foucault's experiments for finding the difference of the velocity 

 of light in air and water. Foucault did not aspire to quantitative pre- 

 cision in his results. The experiments of Michelson proved that the 

 ratio of the velocities was inversely as the indices of refraction. The 

 velocity with sunlight was a little greater than with the electric light; 



