388 PROCEEDINGS OF THE AMERICAN ACADEMY 



which opposes the conclusion of Young and Forbes. When Mr. 

 Michelson covered half of the slit with red glass, the two halves of the 

 imao-e 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 refraction was 1.77, and that orange-red light travelled 

 from one to two per cent faster than greenish blue light. Mr. Mich- 

 elson 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 

 mechanical 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 

 1,640 feet, a disk 25 feet in radius, with 1,000 holes, and turning 360 

 times a second, would be more than sufficient to surprise the reflected 

 ray and stop it. 



In 1874, Burgue suggested a new way 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 arrangement. Two mirrors 

 were placed five meters apart and facing each other. The radius of 

 curvature of each mirror was five meters. The first mirror was 0.20 

 of a meter in diameter : the other, 0.05 m., revolved rapidly (200 

 turns a second). A slit was made in the centre of the large mirror 

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

 the surface of the large mirror : this image became an object for the 

 small mirror, forming another image on the large mirror, at a distance 

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

 images could be sent out laterally by an inclined plate of thin glass, 

 and their distances measured by a micrometer. Wolf expected 

 advantages from the proximity of the two mirrors which would 



