454 mighelson's eecent researches ois^ light. 



The liglit from tlie moving mirror was concentrated on the fixed mir- 

 ror by a lens 8 inches in diameter, with a focal length of 150 feet. These 

 improvements on Foncault's arrangement were so advantageous that 

 Mr. Michelson obtained, even with a smaller speed in the revolving 

 mirror, an angle of separation between the outgoing and returning rays 

 of light so great that the inclined plate of glass in front of the microm- 

 eter was not necessary; the head of the observer not shutting off the 

 light. The mean result of one hundred observations taken on eighteen 

 different days made the velocity of light 180,313 miles per second, with 

 a probable error of 30 miles. 



In 1882, at the request of Professor Newcomb, Mr. Michelson made a re- 

 determination of the velocity of light at the Case Institute, in Cleveland, 

 Ohio, by the method already described, with some modifications. The 

 s[)ace traversed by the light in going and returning between the two 

 mirrors was 4,099 feet. Two slight errors in the reduction of his former 

 work were corrected in this. The velocity deduced from five hundred 

 and sixty-three new observations was 186,278 n)iles, with a probable 

 error of 37 miles. 



In March, 1879, Congress had voted an appropriation of |5,000 for 

 experiments on the velocity of light, to be made under the direction of 

 Professor Newcomb. All the delicacy of instrumental construction, all 

 the skill of scientific observation, and all the resources of mathematical 

 discussion were enlisted in this service. The method adopted was that 

 of the revolving mirror. The movable mirror was mounted at Fort 

 Myer. Two different locations were selected for the fixed mirror, viz, 

 the Naval Observatory and the Washington Monument. In one case 

 the distance was 2,550.95 meters, or about 8,367.12 feet; in the second 

 case, 3,721 meters, or about 12,205.57 feet. Mr. Michelson assisted in 

 the observations until his removal to Cleveland, in the autumn of 1880. 

 The observations began in the summer of 1880, and were continued into 

 the autumn of 1882, the most favorable days in spring, summer, and 

 autumn, being selected. In all five hundred and four sets of measure- 

 ments were made, viz, two hundred and seventy-six by Professor New- 

 comb, one hundred and forty by Professor Michelson, and eighty-eight 

 by Mr. Holcombe. After a full discussion of all the observations and 

 the possible sources of error, Professor Newcomb decided to rest the 

 final result on the one hundred and thirty-two sets of observations made 

 in 1882 over the long distance between Fort Myer and the Washington 

 Monument. The velocity then obtained was 186,282 miles. The ve- 

 locity deduced from the three sets of observations was 186,251 miles. 

 The i)robable error of the first result was about 19 miles. 



For some future attack upon this problem Professor Newcomb sug- 

 gested a prism for the reflector with a pentagonal section, and placed 

 at such a distance that it could revolve through an arc of 36° while the 

 light was going and returning; five hundred turns a second and a dis- 

 tance of 19 miles would fulfill this condition. In the Kocky Mountains, 



