Knowing the number of vibrations of the tuning-fork, and
counting the number of those inscribed upon the paper, it is very simple
to deduce therefrom the amount of the time of exposure. The results of
one of these experiments we have reproduced in Fig. 4. The tuning-fork
gave 100 double vibrations per second. Six vibrations are included
between the opening and closing of the apparatus. Each vibration
estimated at 1/100 of a second. The exposure was 6/100 of a second in
round numbers. This is the amount of the total time of exposure. As
for that of the effective time, that is just as easily ascertained. It
suffices to know the number of vibrations comprised between the moment
at which one point of the objective has been completely uncovered and
that at which it has begun to be covered again. The time is equal to
2/100 in round numbers.
In the experiment in question, with an aperture equal to twice the
diameter of the diaphragm, we have, then, 1/3 of the half-open exposure;
and the amount of the effective time is 1/3.
Pages:
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177