[Illustration: Fig. 2.]
If we cannot obtain such a result in practice, we must nevertheless
aproximate to it. We shall do so by increasing the shutter. Up to C' the
apparatus will operate as before, but from C' to D' the aperture will be
complete, and from D' to B' will decrease as has been said.
Let us give A'B' the same value as AB, that is to say, let us increase
the velocity in the second case in order that the time of exposure shall
be the same; we shall at once see that in the first case the object will
be completely uncovered for only a very short time, while in the second
the exposure will be perfect for a very appreciable period.
The time of exposure which is absolutely active, we propose to call
effective time of exposure in contradistinction to the total time of the
same. The more we increase the value of C'D', that is to say, that of
the effective time, the more the ratio, C'D'/A'B', will approximate to
unity, and the nearer we shall reach perfection.
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