We are now, then, in presence of results that are absolutely
positive, and they are as follows:
1. The guillotine shutter should be placed in the interior of the
objective and as near as possible to optical center, that is to say,
behind the diaphragm, since the latter is precisely in the optical
center.
2. The aperture should be as wide as possible.
3. The velocity should be as great as possible.
In practice, an aperture from 4 to 5 times the diameter of diaphragm
employed will be more than sufficient, since we shall have, according to
circumstances, 3/4 or 4/5 of the effective time. Moreover, whatever be the
time of exposure, this ratio once established will be invariable, and
the apparatus will always operate identically.
A shutter combining these qualities will not yet be perfect. It is
necessary, according to the time and the light, that the time of
exposure shall be capable of being varied. In a word, it is necessary
that the apparatus shall be _graduated_ and permit of taking views more
or less quickly.
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