392 A. A. Michelson — Velocity of Light. 
feet, and the speed of the mirror ‘was about 257 revolutions 
per second. The deflection exceeded 183 millimeters, being 
about 200 times as great as that obtained by Foucault. If it 
were necessary it could be still further increased. This deflec- 
tion was measured within three or four hundredths of a milli- 
meter in each observation ; and it is safe to say that the result, 
so far as it is affected by this measurement, is correct to within 
one ten-thousandth part. . 
The revolving mirror was actuated by a current of air which 
escaped through a turbine wheel on the same axle as the 
i 
and measure the speed of rotation a tuning-fork, bearing on 
Hence, to make the mirror revolve at a given uniform speed, 
the cord attached to the valve, which leads to the observer's 
table, is pulled right or left, till the images of the revolving 
mirror come to rest. 
The electric fork made about 128 vibrations per second. No 
dependence was placed upon this rate, however, but at each 
set of observations it was compared with a standard Ut, fork, 
the temperature being noted at the time. 
The rate of the Ut, fork was found to be 256°072 at 65° F. 
The result obtained by Prof. Mayer and myself, at the Stevens 
Institute, was 256-068. 
apparatus for measuring the deflection consists of an 
accurate screw with divided circle. To the frame is attached 
an adjustable slit. On the screw travels a carriage which 
supports the eye-piece, which consists of an achromatic lens, 
_ having in its focus a single vertical silk fiber. The slit which 
_ 38 very nearly in the same focal plane as the silk fiber, 18 
bisected by the latter, and reading of scale and circle taken. 
_ Then the screw is turned till the silk fiber bisects the deflect 
_ Image of the slit, and reading taken again. The difference 
veen the two readings gives the deflection. 
