Let us examine and discuss what occurs in the three cases.
Suppose a rectilinear objective of the kind most usually employed in
instantaneous photography, and an object, A B, that we wish to reproduce
(Fig. 1), the objective being provided with any sort of diaphragm. The
point, A, sends a bundle of rays, a"b", to the first lens. Here they are
slightly refracted, and then go on parallel lines to the second lens,
where they are again refracted and form at A' an image of A. It is this
image that we see upon the ground glass, and which makes an impression
upon the sensitive film. The point, B, behaves in the same way and
gives an image at B', but, as will be at once seen, the image will be
reversed. In our figure, A corresponds to the sky and B to the earth.
If, then, the shutter passes in front of the objective, it will first
allow of the passage of the rays which come from the sky, then, on
continuing its travel, it will unveil the landscape, and lastly the
ground. As it is submitted to the law of the fall of bodies and has a
uniformly increasing velocity, it follows that the time of exposure will
uniformly decrease between A' and B', and that the sky will pose longer
than the foreground.
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