The dimensions of the first will be
practical, and those of the second will give too cumbersome and too
fragile an apparatus. But why must the aperture be larger than the
diaphragm employed? This is what we are going to demonstrate. Let us
make the aperture equal to the diameter of the objective, and see
what occurs at the different periods of the exposure. For the sake of
clearness, we shall suppose the velocity uniform.
It is evident, _a priori_, that a perfect apparatus will be the one that
will allow the light to act during the entire exposure with a maximum of
intensity. Is it thus, when the aperture is equal to the diameter of the
objective? Evidently not. Let us consult Fig. 2. We here see the shutter
progressively uncovering the objective. The light will increase from A
to C up to the moment when the objective is entirely uncovered, and will
then immediately decrease up to B. The objective has operated with a
maximum of light for only a short time. We are far from the ideal result
in which the maximum of light, CD, should exist during the entire
exposure, and form the upper plane precisely equal to AB.
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