The sinking of the caissons of the Forth Bridge has afforded
opportunity for testing whether it was possible to obtain photographs
below water in compressed air by the aid of electric light. To the
novel conditions under which these attempts—the first, we believe, in
this country—were made, their chief interest is due, rather than to any
particular success hitherto achieved. We have recently described the
method of founding by compressed air, and depicted the interior of a
caisson, so that our readers are conversant with the surroundings under
which the attempts were made.
A trial was made on shore by electric light at night to determine the
length of exposure necessary for the plates; but subsequent experience
proved the data thus obtained to be of little value in the air-chamber.
Various trials were then made in the air-chamber with different classes
of plates and gradually increasing lighting-power; eventually, five
arc-lamps—each equivalent to twelve hundred candles—and plates of
exceptional rapidity, were employed; and these, with an exposure of two
minutes, gave the best results obtained.
The roof and sides of the air-chamber were whitewashed, to render them
conspicuous and to diffuse the light. The formation of moisture on the
lens threatened at first to give trouble; but after a little time
the glass became warmed, and the difficulty ceased. The haze in the
air-chamber, which any sudden expansion of the air—such as that due to
its escape when the air-locks were opened—greatly intensified, proved
a formidable obstacle, and must always render the highest results
unattainable. The only course was to seize the most favourable moment
when the haze was at its minimum. White objects and light clothing gave
the best results; whilst the eyes of a group—presumably from their
glistening properties—are remarkable for definition and sharpness.
So far as could be ascertained, no injury resulted to the dry plates
either from air-pressure or moisture.