_Mr. Warnerke's New Discovery_.--Very happily for our art, we are at the
present moment entering upon a stage of improvement which shows that
photography is advancing with vast strides toward a position that has
the possibility of a marvelous future. In England, especially, great
advances are being made. The recent experiments of our accomplished
colleague, Mr. Warnerke, on gelatine rendered insoluble by light, after
it has been sensitized by silver bromide and developed by pyrogallic
acid, have revealed to us a number of new facts whose valuable results
it is impossible at present to foretell. It seems, however, certain that
we shall thus be able to accomplish very nearly the same effects as
those obtained by bichromatized gelatine, but with the additional
advantage of a much greater rapidity in all the operations. In my own
experiments with the new process of phototypie, I hit upon the plan of
plunging the carbon image, from which all soluble gelatine had been
removed, into a bath of pyrogallic acid, in order to still further
render impermeable the substance forming the printing surface. I also
conceived the idea of afterward saturating this carbon image with a
solution of nitrate of silver, and of subsequently treating it with
pyrogallic acid, in order to still further render impermeable the
substance forming the printing surface. But the process described by Mr.
Warnerke is quite different; by means of it we shall be able to fix
the image taken in the camera, in the same way as we develop carbon
pictures, and afterward to employ them in any manner that may be
desirable. Thus the positive process of carbon printing would be
modified in such a manner that the mixtures containing the permanent
pigment should be sensitized with silver bromide in place of potassium
bichromate. In this way impressions could be very rapidly taken of
positive proofs, and enlargements made, which might be developed in hot
water, just as in the ordinary carbon process, and at least we should
have permanent images. Mr. Warnerke's highly interesting experiments
will no doubt open the way to many valuable applications, and will
realize a marked progress in the art of photography.
_Method for Converting Negatives Directly into Positives_.--Captain
Bing, who is employed in the topographic studios of the Ministry of
War, has devised a process for the direct conversion of negatives into
positives. The idea is not a new one; but several experimenters, and
notably the late Thomas Sutton, have pointed out the means of effecting
this conversion; it has never, however, so far as I know, been
introduced into actual practice, as is now the case. The process which
I am about to describe is now worked in the studios of the Topographic
Service. The negative image is developed in the ordinary way, but the
development is carried much further than if it were to be used as an
ordinary negative. After developing and thoroughly washing, the negative
is placed on a black cloth with the collodion side downward, and exposed
to diffuse light for a time, which varies from a few seconds to two or
three minutes, according to the intensity of the plate. Afterward the
conversion is effected by moistening the plate afresh, and then plunging
it into a bath which is thus composed:
Water 700 cub. cents.
Potassium bichromate 30 grams.
Pure nitric acid 300 cub. cents.
In a few minutes this solution will dissolve all the reduced silver
forming the negative; the negative image is therefore entirely
destroyed; but it has served to impress on the sensitive film beneath
it a positive image, which is still in a latent condition. It must,
therefore, be developed, and to do this, the film is treated with a
solution of--
Water 1,000 grams
Pyrogallic acid 25 "
Citric acid 20 "
Alcohol of 36° 50 cub. cents.
The process is carried on exactly as if developing an ordinary negative;
but the action of the developer is stopped at the precise moment when
the positive has acquired intensity sufficient for the purpose for which
it is to be used. Fixing, varnishing, etc., are then carried on the
usual way. The great advantage of this process consists in the fact of
its rendering positives of much greater delicacy than those that are
taken by contact; and, on the other hand, by means of it we are able to
avoid two distinct operations, when for certain kinds of work we require
positive plates where a negative would be of no service. M. V. Rau,
the assistant who has carried out this process under the direction of
Captain Bing, has described it in a work which has just been published
by M. Gauthier-Villars.
_Experiments of Captain Bing on the Sensitiveness of Coal Oil_.--The
same Captain of Engineers has undertaken a series of very interesting
experiments on the sensitiveness to light of one or two substances to
which bitumen probably owes its sensitiveness, but which, contrary to
what takes place with bitumen, are capable of rendering very beautiful
half tones, both on polished zinc and on albumenized paper. These
sensitive substances are extracted by dissolving marine glue or coal-tar
in benzine. By exposure to light, both marine-glue and coal-tar turn of
a sepia color, and, in a printing-frame, they render a visible image,
which is not the case with bitumen; their solvents are in the order of
their energy; chloroform, ether, benzine, turpentine, petroleum spirit,
and alcohol. Of these solvents, benzine is the best adapted for reducing
the substances to a fluid state, so as to enable them to flow over the
zinc. The images obtained, which are permanent, and which are very much
like those of the Daguerreotype, are fixed by means of the turpentine
and petroleum spirit. They are washed with water, and then carefully
dried. It is possible to obtain prints with half-tones in fatty ink by
means of plates of zinc coated with marine-glue. Some attempts in this
direction were shown to me, which promised very well in this respect. We
are, therefore, in the right road, not only for economically producing
permanent prints on paper, but also for making zinc plates in which the
phototype film of bichromatized gelatine is replaced by a solution of
marine-glue and benzine. The substance known in commerce under the name
of pitch or coal-tar will produce the same results.
_Bitumen Plates_.--A new method of making bitumen plates by contact has
also been introduced into the topographical studios. The plan, or the
original drawing, is placed against a glass plate, coated with a mixture
of bitumen and of marine-glue dissolved in benzine. The marine-glue
gives the bitumen greater pliancy, and prevents it from scaling off when
rubbed, particularly when the plate is retouched with a dry point.
These bitumen plates are so thoroughly opaque to the penetration of the
actinic rays, that the printing-frame may be left for any time in full
sunlight without any fear of fog being produced on the zinc plate from
which the prints are to be taken.
_Method for Topographic Engraving by Commandant de la Noë_.--Before
leaving the interesting studios of which I have been speaking, I ought
to mention a very ingenious application which has been made of a process
called _topogravure_, invented by Commandant de la Noë, who is the
director of this important department. A plate of polished zinc is
coated with bitumen in the usual way, and then exposed directly to the
light under an original drawing, or even under a printed plan. So soon
as the light has sufficiently acted, which may be seen by means of
photometric bands equally transparent at the plate, all the bitumen not
acted upon is dissolved. As it is a positive which has acted as matrix,
the uncovered zinc indicates the design, and the ground remains coated
with insoluble bitumen. The plate is then etched with a weak solution
of nitric acid in water, and the lines of the design are thus slightly
engraved; the surface is then re-coated with another layer of bitumen,
which fills up all the hollows, and is then rubbed down with charcoal.
All the surface is thus cleaned off, and the only bitumen which remains
is that in the lines, which, though not deep, are sufficiently so to
protect the substance from the rubbing of the charcoal. When this
is done we have an engraved plate which can be printed from, like a
lithographic stone; it is gummed and wetted in the usual way, and it
gives prints of much greater delicacy and purity than those taken
directly from the bitumen. The ink is retained by the slight projection
of the surface beyond the line, so that it cannot spread, and a kind of
copper plate engraving is taken by lithographic printing. Besides, in
arriving at this result, there is the advantage of being able to use
directly the original plans and drawings, without being obliged to have
recourse to a plate taken in the camera; the latter is indispensable
for printing in the usual way on bitumen where the impression on the
sensitive film is obtained by means of a negative. It will be seen that
this process is exceedingly ingenious, and not only is its application
very easy, but all its details are essentially practical.
_Succinate of Iron Developer_.--I have received a letter from M.
Borlinetto, in which he states that he has been induced by the analogy
which exists between oxalic and succinic acids to try whether succinate
of iron can be substituted for oxalate of iron as a developer. To prove
this he prepared some proto-succinate of iron from the succinate of
potassium and proto-sulphate of iron, following the method given by Dr.
Eder for the preparation of his ferrous oxalate developer. He carried
out the development in the same way as is done by the oxalate, and
he found that the succinate of iron is even more energetic than the
oxalate. The plate develops regularly with much delicacy, and gives a
peculiar tone. It is necessary to take some fresh solution at every
operation, on account of the proto-succinate of iron being rapidly
converted into per-succinate by contact with the air.
_Method of Making Friable Hydro-Cellulose_.--At the meeting of the
Photographic Society of France, M. Girard showed his method of preparing
cellulose in a state of powder, specially adapted for the production of
pyroxyline for making collodion. Carded cotton-wool is placed in water,
acidulated with 3 per cent. of sulphuric or nitric acid, and is left
there from five to fifteen seconds; it is then taken out and laid on a
linen cloth, which is then wrung so as to extract most of the liquid. In
this condition there still remains from 30 to 40 per cent. of acidulated
water; the cotton is divided into parcels and allowed to dry in the open
air until it feels dry to the touch, though in this condition it still
contains 20 per cent. of water. It is next inclosed in a covered jar,
which is heated to a temperature of 65° C.; the desiccation therefore
takes place in the closed space, and the conversion of the material
is completed in about two or three hours. In this way a very perfect
hydro-cellulose is obtained, and in the best form for producing
excellent pyroxyline.--_Corresp. Photo Mews_.
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