At the last meeting of the Chemical Society Captain Abney gave a lecture
on the above subject to a large audience. We may premise by saying that
the demonstrations he gave were carried out principally by means of
experiments on paper, to enable his hearers to understand the different
points he wished to enforce. The lecture was commenced by insisting on
the fact that all photographic action took place within the molecules of
the compound acted upon and not on the molecule itself, and from this
he deduced that the absorption of radiation which take place by such
compounds is principally caused by the atoms composing the molecule.
This was found to be the case in the organic liquids, which the lecturer
to some extent had investigated, where he had further traced the
absorption to the vibrating atoms of hydrogen in those bodies. In
order to properly investigate the action of light it was necessary to
ascertain which components of light in the spectrum were the chief
agents in causing it, and this led him to consider the means to be
employed to obtain a spectrum.
The effects of diffraction gratings were first discussed, and in two
which were shown it was found that in some spectra the visible portions
were dimmed; in others the ultra-violet and the infra-red were almost
entirely absent. It thus became necessary to investigate the condition
of a grating before placing any confidence in the results obtained. This
was the first pitfall into which an experimentalist was liable to fall.
If prisms were used for obtaining the spectrum, then precautions had
also to be taken, since all glass absorbed a portion of the ultra-violet
rays and some the infra-red. On the whole, he considered that the best
glass to use was pure white flint glass for the collimator, the prisms,
and the camera lens. Another inquiry that was necessary was the
source of radiation which it was proposed to use. Diagrams showed the
unsatisfactory nature of solar radiation, and a photograph of the whole
spectrum, taken with it under certain atmospheric conditions in which
the effect of the green rays were almost _nil_, demonstrated the false
conclusions that might be deduced as to the sensitiveness of any
particular compound.
Captain Abney also showed the satisfactory conditions which existed in
using the crater of the positive pole of the electric arc light as a
source, and by diagrams illustrated the inferiority of an incandescent
light for the purpose, owing to the deficiency of violet and
ultra-violet rays. Having thus settled the source of illumination and
the kind of apparatus to employ, he next considered the conditions under
which the sensitive salts were to be exposed. The action of ordinary
sensitizers was explained and demonstrated by experiments, from which
point the results of certain colored sensitizers were considered. Thus,
various aniline dyes were proved to be bromine absorbents, and likewise,
more or less, to be capable of being acted upon by light in those
regions of the spectrum they absorbed. The result of the two effects was
to produce a developable image of the spectrum just in those parts to
which the salt of silver was sensitive, and also in the parts where the
dye itself was acted upon. The latter effect was traced to the organic
matter being oxidized in the presence of the sensitive silver salt.
The sensitizing effect of one silver compound upon another was then gone
into, and experiments and photographs showed where two salts of silver
were in contact with one another, and without an energetic sensitizer
being at hand, that the one when acted upon by light absorbed the
halogen liberated from the other through the same cause and that a new
molecule was formed. This was of importance, since in photographic
spectroscopic researches a conclusion might be arrived at that a
body suffered absorption in those regions of the spectrum where this
interesting reaction took place, whereas in reality the phenomenon might
be due to the silver salts employed. This was another pitfall for the
unwary. Again, it became necessary in studying photographic action to
make sure that the effect of radiation was only a reducing action, and
that the results were not vitiated by some other action.
The destruction by oxidizing agents of the effect produced by light was
then experimentally demonstrated, and photographs of the spectrum showed
that this effect was increased by the action of light itself. Thus, when
immersing a plate sensitive to all radiations, visible and invisible,
in a very dilute solution of nitric acid, bichromate of potash, or
hydroxyl, it was shown that if the plate were exposed to light, first
the parts acted upon by the red rays were reduced before the parts not
acted upon at all by the spectrum, thus conclusively proving that light
itself helped forward the oxidation or so-called solarization of the
image. It thus became a struggle, under ordinary circumstances, between
the reducing action on the normal salt and the oxidizing action on the
altered salt as to which should gain the mastery. If the reducing action
of any particular ray were the most active, then a negative image
resulted, whereas if the oxidizing action were in the ascendant, a
positive image resulted. Thus, in determining the action of light on
a particular salt, this antagonism had to be taken into account, and
exposure made with such precautions that no oxidizing action could
occur, as would be the case if an inorganic sensitizer, such as sulphite
of soda, were used.
The reversal of the image by soluble haloid salts, such as bromide of
potassium, was then dwelt upon with experimental demonstration. It was
shown that the merest trace of soluble haloid would reverse an image
by the extraction of bromine from it, and the fact that the most
refrangible part of the spectrum was principally efficacious in
completing this action showed how necessary it was to avoid falling
into error when analyzing photographic action by the spectroscope. A
reference was next made to gelatine plates, in which, owing to their
preparation, reversal through the above cause was most likely to take
place, and a plate soaked in sulphite of soda and exposed in the camera
for a couple of minutes--a time largely in excess of that necessary to
give a reversal under ordinary circumstances--proved the efficacy of
the oxygen absorber, the image remaining in its normal condition after
development.
The lecturer closed his remarks by showing the different molecular
states of iodide, bromide, and chloride of silver, as produced by
different modes of preparation. The color of the film by transmitted
light in every case indicated the effect which was likely to be produced
on them, and the photographed spectrum in each of them showed the
remarkable differences that were found. The points raised by Captain
Abney at different times are well worthy the study of scientific
photographers, since strict attention to the modes of exposure to the
spectrum, to the instruments employed, and to the source of light used
can alone insure accuracy in comparative experiments.--_Br. Jour. of
Photo_.
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