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, ¾ 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. 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. 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. The difference that we
have in practice is due to the fact that the velocity is uniformly
accelerated. In order to increase the amount of the effective time, it
will be only necessary to increase the aperture of the shutter and apply
again the method that we have just pointed out.
[Illustration: FIG. 3.]
So much for the material part of the apparatus. It will be necessary
in addition to acquire sufficient individual experience to be able to
estimate the intensity of the light, and consequently to judge of the
diaphragm to be employed and the velocity to be obtained. It must not
be forgotten that such or such an object having a relatively slow speed
will not be sufficiently sharp on the negative if it is too near
the apparatus, while such or such another, much more rapid, might
nevertheless be caught if sufficient distance intervened. Here it is
that will appear the skill of the amateur, who will find it possible to
obtain the said object as large as possible and with a maximum degree of
sharpness.
We have seen what diverse qualities should be possessed by a good
guillotine shutter, and it is evident that the same should be found
in all apparatus of the kind. In our opinion the guillotine is a well
defined type that possesses one capital advantage, and that is that it
permits of the use of aperatures as wide as may be desired for the
same time of exposure. It is a question, as we have seen, of velocity.
Consequently, however short the exposure be, it will always be possible
to operate with a full amount of light during the greater part of the
exposure. It is necessary to dwell upon this point, since in another
kind of apparatus that possesses a closing and opening shutter the same
result cannot be reached. In the Boca apparatus, for instance, we remark
that at a given moment the time of exposure is reduced to nothing, as
the closing shutter covers the objective before the latter has been
unmasked by the opening one. In all exposures, in fact, the times of
opening and closing have a constant value. It follows that the shorter
the exposure is, the greater becomes such value, and to such a point
that, at a given moment, the apparatus no longer make an exposure.
[Illustration: FIG. 4.]
In the guillotine, on the contrary, the same space always intervenes
between the time of opening and closing, since it is fixed in an
unvarying manner by the diameter at the aperature. Then, the greater the
velocity, the more the time of opening and closing diminishes. If
the ratio of the effective to the total time of exposure is 3/4, for
example, it will be invariable, whatever be the velocity.
In concluding, we will remark that, without employing springs, we
may increase the aperture of the shutter without varying the time of
exposure. To effect this it is only necessary to raise the point of the
shutter's drop. In fact, as may be seen in Fig. 4, all the vibrations
of the stylet corresponding to 1/100 of a second always continue to
elongate, and it will consequently be possible for the same time of
exposure to considerably increase the aperture and, as a consequence,
the effective time, by causing the guillotine to drop from a greater
elevation. From this study, which has principally concerned the
guillotine shutter, can we draw the deduction that this type of
apparatus will become a definite one? We think not. In fact, along with
its decided advantages the guillotine has a few defects that cannot be
passed over in silence. The aperture, in measure as it is increased,
renders the apparatus delicate and subject to become bent. If, in order
to obviate this trouble, we employ plates of steels, we increase its
weight considerably, and the chamber becomes subject to vibration at the
moment the shutter drops. If rubber or springs are used for increasing
the velocity, it is still worse. Moreover, it is quite difficult to
obtain a graduation, and to our knowledge, and probably for this reason,
it has not yet been applied.
The reader will please excuse us for this perhaps somewhat dry
theoretical _expose_, but we have thought it well to give it in the
hope that it might well show the qualities that should be required of a
photographic shutter and particularly of the guillotine. Moreover, at
the point to which photography has arrived it is no longer permitted to
do things by halves.
After the memorable discoveries of Nicephore, Niepce, Daguerre, and
Talbot, photography remained for some time stationary, limited to the
production of portraits and landscapes. But for a few years past it has
taken a new impetus, and new processes have come to the surface. In the
graphic arts and in the sciences it has taken considerable place. Being
the daughter of chemistry and physics, it is not astonishing that we
require of it the precision of both. It is, moreover, through a profound
study of the reactions that gave it birth and through a knowledge of the
laws of optics that it has come into current use in laboratories. In
fact, it alone is capable of giving with an undoubted character of
truthfulness a durable vestige of certain fleeting phenomena.--
_A. Londe, in La Nature_.
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