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.
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