Evening star. [volume] (Washington, D.C.) 1854-1972, 1910-02-27
Do You Know? P Psfh.e: You YOU B Do

plate without use of the intermediate mega
tive. This is brought about by the phenomenon
called reversal" If plate is exposed suffi-
cient length of time, that when it is printed
and lights. normal exposure is said to
have been produced. This normal exposure
when in midday sunlight. using the largest
stop in the camera, in general takes anywhere
of second.
If plate is exposed for longer time. an
abnormal exposure is obtained If a series of
abnormal exposures are TAKEN which have been
exposed respectively for one, ten. twenty.
thirty, and forty-fve seconds, one minute, and
fifteen minutes, and then these plates de-
veloped it is found that in the one-second plate
the lights are less contrasted with the shadows
than in the normal plate In the ten-second
plate the effect increases; until in the twenty
second plate a black, or logged, negative with
almost complete lack of contrast is produced
In the thirty second plate, however, the lights
of the original object photographed begin to APP
pear light. And the longer overexposures
developed it is seen that there is reversal
to positive in parts of the plate. In fifteen
minute exposure this reversal becomes COM
plete and an entire positive plate is obtained
This positive plate, however, is weaker all over
than is the normal negative. From positive
plate negative prints may be made which give
a curious effect.
Suppose that a plate is exposed for several
weeks, instead of for a ffteen-minute period ;
in this case a second reversal partially takes
place. and the plate when developed shows
negative in some parts, while other parts still
remain the positive of the first reversal. 1t is
difficult to get this second reversal complete
and it is doubtful if perfect one could be OBJ
tanned.
What causes the phenomenon of reversal is
a case of conjecture: but one of the better
hypothesis in explanation of this fact has been
advanced by German named Luther. In the
first place it must be considered that a photo
graphic plate has spread on its surface film of
gelatine which contains silver bromide. When
the light strikes the silver bromide, it decom-
poses it and bromine is given off. The gelatine
absorbs this bromine, and thereby becomes
slightly harder. The longer the film is exposed
to the light, the more bromine is given off
hence it follows that the more bromine the
gelatine absorbs the harder the gelatine is sure
to become.
When plate is developed, the parts of it
struck by light are turned black by the devel-
oper. Inasmuch as the development of plate
depends on the permeability of the film to the
developer in plate that has been exposed for
fifteen minutes the gelatine that corresponds to
the lights has been SO hardened by the absorb
ton of bromine that it is almost impervious to
the developer, and hence no blackness appears
The gelatine on the shadows however (Which
have been exposed long enough to equal the
lights in a normal plate) is less hard allowing
the developer to soak in and thereby develop
the shadows black. Thus the final effect is
that of light lights and dark shadows. or, in
short. positive.
The explanation of the second reversal is
along similar lines; but is more compiled ted,
and therefore understood only by one well
versed in the subject -HoYold r. Rafsky