The different velocities might be given to the
shutter by means of weights, rubber, or springs. The latter seem to be
preferable, since they permit in the first place of operating out of
the vertical; moreover, they are less fragile, and, through different
tensions, they permit of these graduations that we consider as
indispensable. For the current needs of practice 1/100 of a second is a
limit that seems to us sufficient as a maximum of rapidity. In order to
know the time of exposure obtained we employ the following method, which
permits of graduating an apparatus rapidly and with extreme precision:
A band of smoked paper is fixed upon the shutter, then a tuning-fork
provided with a small stylet resting against the paper is made to
vibrate. Better yet, a chronograph which vibrates synchronously with a
tuning-fork, whose motion is kept up by electricity, is put in the same
place. Fig. 3 shows the arrangement to be employed. We then let the
shutter fall, when the little stylet will inscribe a certain number of
vibrations.
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