45() michelson's recent kesearches on light. 



and the differeuoe of time for air and water woiilil be only a fraction of 

 that small fraction. Hence the exceeding delicacy of the experiment. 



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

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

 light in air and water. Foncault did not aspire to quantitative precis- 

 ion 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; which 

 oi)poses the conclusion of Young and Forbes. When Mr. Michelson 

 covered half of the slit with red glass, the two halves of the image were 

 exactly in line. Experiments were also made on the velocity of light 

 in carbon disulphide, which led to the inference that its index of refrac- 

 tion was 1.77, and that orange-red light traveled from one to two i)er 

 cent, faster than greenish blue light. Mr. Michelson was enabled to 

 make this investigation by a grant from the trustees of the -Bache Fund. 



Various other methods of measuring the velocity of light have been 

 proposed. About 1850, Laborde suggested, in a letter to Arago, a me- 

 chanical method of measuring the velocity of light. He supposes two 

 disks, with many holes at the outside, connected by a very long axis 

 and rotating rapidly. The light which was sent out through a hole in 

 one wheel would be transmitted or arrested by the second wheel, behind 

 which an observer was stationed. The distance between the wheels, 

 the time of rotation, and the order of the eclipse, would be sufficient for 

 calculating the velocity of light. Laborde imagined an enormous axis 

 more than 200,000 miles long. Moigno recommended the substitution 

 of a mirror for the observer and the second, wheel, which would double 

 the distance travelled by the light. A distance of 1,040 feet, a disk 25 

 feet in radius, with 1,000 holes, and turning 300 times a second, would 

 be more than sufficient to surprise the reflected ray and stop it. 



In 1871, Burgue suggested a new wa}' of finding the velocity of light 

 by experiment. If a white disk, with a black radius, is rotated rapidly, 

 and at each turn is illuminated by an instantaneous flash, this radius 

 will appear immovable. If this flash is reflected on the disk from a 

 distant mirror, the black radius will be displaced. No details of the 

 arrangement of apparatus and no experiments were published. 



In 1885, Wolf proposed the following arrangements: Two mirrors 

 were placed 5 meters apart and facing each other. The radius of curv- 

 ature of each mirror was 5 meters. The first mirror was 0.20 of a meter 

 in diameter ; the other, 0.05 meter, revolved ra])idly (two hundred turns 

 a second), A slit was made in the center of the large mii-ror through 

 which light was sent to the small mirror, forming an image on the sur- 

 face of the large mirror ; this image became an object for the small 

 mirror, forming another image on the larger mirror, at a distance from 

 the first mirror depending on the velocity of rotation. These images 

 could be sent out laterally by an inclined i)late of thin glass, and their 

 distance measured by a micrometer. Wolf expected advantages from 



