both at rest, an image of the revolving mirror is seen. When the fork
vibrates, this image is drawn out into a band of light.

When the mirror commences to revolve, this band breaks up into a number of
moving images of the mirror; and when, finally, the mirror makes as many
turns as the fork makes vibrations, these images are reduced to one, which
is stationary. This is also the case when the number of turns is a
submultiple. When it is a multiple or simple ratio, the only difference is
that there are more images. Hence, to make the mirror execute a certain
number of turns, it is simply necessary to pull the cord attached to the
valve to the 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 is
compared with a standard Ut₃ fork, the temperature being noted at the
same time. In making the comparison the sound-beats produced by the forks
were counted for 60 seconds. It is interesting to note that the electric
fork, as long as it remained untouched and at the same temperature, did
not change its rate more than one or two hundredths vibrations per second.

[Illustration: FIG. 9.]



The Observer's Table.


Fig. 9 Represents The Table At Which The Observer Sits. The Light From The
Heliostat Passes Through The Slit At S, Goes To The Revolving Mirror, &c.,