michelson's recent researches on light. 455 



or tbo Sierra Nevada, stations from 20 to 30 miles distant could be 

 found, and with no greater loss of light from absorption tlian is ])ro- 

 duced by 2 or 3 miles of common air. 



The first experiments made in Great Britain for tbe measurement of 

 the velocity of light were published by James Young and Prof. G. 

 Forbes in the Philosophical Transacfions of 1883. They adopted the 

 method of Fizeau. In 1878, between six and seven hundred observa- 

 tions were made; but the number of teeth in the rotating wheel was 

 insufiScient. Il^ew experiments were made in 1880-'81 across the river 

 Clyde. Two reflectors were used at unequal distances, and the time 

 was noted when an electric light after the two reflections was at its 

 maximum. The corrected distances for the two mirrors were 18,212.2 

 and 16,835 feet. After an elaborate mathematical discussion of the 

 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 different ex- 

 l^erimeuts. Mr. Young and Professor Forbes made some experiments 

 with lights of different colors, in confirmation of this view. But Profes- 

 sor Michelson compared his three hundred and eighteen observations 

 with sunlight and two hundred and sixty-seven observations with 

 electric light, and found tliat the difference was in the opposite direc- 

 tion; 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 electro-magnetic theory of light, and also as promis- 

 ing 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 experi- 

 ment 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 different 

 classes of phenomena in geometrical optics can be explained by either 

 one of these theories, though even here the undulatory has the advan- 

 tage of greater simplicity. But in one respect the two theories are an- 

 tagonistic. According to the corpuscular theory, light should move 

 faster in glass or water than in air, for example. The undulatory theory 

 reversed this proposition. Ilere was an experiment ion crucis. In 1850, 

 Fizeau and Foucault made the experiment, eacli 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 thou- 

 sand feet are i)racticable. But the absorbing power of water prevents 

 the use of greater lengths than about 1 feet. Ligh t would pass through 

 10 feet of air in less time than one eighteen-thousandth of a second: 



