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development is conducted in a flat dish, sometimes the solution is poured on the plate. The unreduced salts are eliminated by either cyanide of potassium or sodium hyposulphite. Intensity may be given to the image, if requisite, either before or after the "fixing" operation. Where resort is had to ferrous oxalate development, the developer is made in one of two ways-(1) by saturating a saturated solution of neutral potassium oxalate with ferrous oxalate, and adding an equal volume of a solution (10 grains to I oz. of water) of potassium bromide to restrain the action, or (2) by mixing, according to Eder's plan, 3 volumes by measure of a saturated solution of the potassium oxalate with I volume by measure of a saturated solution of ferrous sulphate, and adding to the ferrous oxalate solution thus obtained an equal bulk of the above solution of potassium bromide. The development is conducted in precisely the same manner as indicated above, and the image is fixed by one of the same agents.

Gelatin Emulsion Process.-The facility with which silver bromide emulsion could be prepared in collodion had turned investigation into substitutes for it. As early as September 1871 Dr R. L. Maddox had tried emulsifying the silver salt in gelatin, and had produced negatives of rare excellence. In November 1873 J. King described a similar process, getting rid of the soluble salts by washing. Efforts had also been made in this direction by J. Burgess in July 1873. R. Kennett in 1874 may be said to have been the first to put forward the gelatin emulsion process in a practical and workable form, as he then published a formula which gave good and quick results. It was not till 1878, however, that the great capabilities of silver bromide when held in suspension by gelatin were fairly known; in March of that year C. Bennett showed that by keeping the gelatin solution liquid at a low temperature for as long as seven days extraordinary rapidity was conferred on the sensitive salt. The molecular condition of the silver bromide seemed to be altered, and to be amenable to a far more powerful developer than had hitherto been dreamt of. In 1874 J. S. Stas had shown that various modifications of silver bromide and chloride were possible, and it seemed that the green molecular condition (one of those noted by Stas) of the bromide was attained by prolonged warming. It may be said that the advent of rapid plates was 1878, and that the full credit of this discovery should be allotted to C. Bennett. Both Kennett and Bennett got rid of the soluble salts from the emulsion by washing; and in order to attain success it was requisite that the bromide should be in excess of that necessary to combine with the silver nitrate used to form the emulsion. In June 1879 Abney showed that a good emulsion might be formed by precipitating a silver bromide by dropping a solution of a soluble bromide into a dilute solution of silver nitrate. The supernatant liquid was decanted, and after two or three washings with water the precipitate was mixed with the proper amount of gelatin. D. B. van Monckhoven of Ghent, in experimenting with this process, hit upon the plan of obtaining the emulsion by acting on silver carbonate with hydrobromic acid, which left no soluble salts to be extracted. He further, in August 1879, announced that he had obtained great rapidity by adding to the bromide emulsion a certain quantity of ammonia. This addition rapidly altered the silver bromide from its ordinary state to the green molecular condition referred to above. At this point we have the branching off of the gelatin emulsion process into two great divisions, viz. that in which rapidity was gained by long-continued heating, and the other in which it was gained by the use of ammonia-a subdivision which is maintained to the present day. Opinions as to the merits of the two methods are much divided, some maintaining that the quality of the heated emulsion is better than that produced by alkalinity, and vice versa. We may mention that in 1881 Dr A. Herschel introduced a plan for making an alcoholic gelatin emulsion with the idea of inducing rapid drying of the plates, and in the same year H. W. Vogel of Berlin introduced a method of combining gelatin and pyroxylin together by means of a solvent which acted on the gelatin and allowed the addition of alcohol in order to dissolve the pyroxylin. This "collodio-gelatin emulsion" was only a shortlived process, which is not surprising, since its preparation involved the inhalation of the fumes of acetic acid.

The warming process introduced by Bennett was soon superseded. Colonel Stuart Wortley in 1879 announced that, by raising the temperature of the vessel in which the emulsion was stewed to 150° F., instead of days being required to give the advance was made by boiling the emulsion, first practised, we desired sensibility only a few hours were necessary. A further believe, by G. Mansfield in 1879. Another improvement was effected by W. B. Bolton by emulsifying the silver salt in a small quantity of gelatin and then raising the emulsion to boiling point, boiling it for from half an hour to an hour, when extreme rapidity was attained. Many minor improvements in this process have been made from time to time. It may be useful to give an idea of the relative rapidities of the various processes we have described.

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The following is an outline of two representative processes. All operations should be conducted in light which can act but

very slightly on the sensitive salts employed, and this is more necessary with this process than with others on account of the upset in giving rise to the molecule which is developable. extreme ease with which the equilibrium of the molecules is The light to work with is gaslight or candlelight passing through a sheet of Chance's stained red glass backed by orange paper. Stained red glass allows but few chemically effective rays to pass through it, whilst the orange paper diffuses the light. If of orange paper. The following should be weighed out:daylight be employed, it is as well to have a double thickness

I.

Potassium iodide

Potassium bromide

Nelson's No. I photographic gelatin
Silver nitrate

2.

3.

4.

5.

Nelson's No. I gelatin

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Autotype or other hard gelatin

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5 grs.

135 "

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Nos. 3 and 5 are rapidly covered with water or washed for a few seconds under the tap to get rid of any dust. No. 2 is dissolved in 1 oz. of water, and a little tincture of iodine added till it assumes a light sherry colour. No. I is dissolved in 60 minims of water. No. 4 is dissolved in oz. of water, and No. 3 is allowed to swell up in I oz. of water, and is then dissolved by heat. All the flasks containing these solutions are placed in water at 150° F. and carried into the called; Nos. 3 and 4 are then mixed together in a jar or flask, and dark room," as the orange-lighted chamber is ordinarily No. 2 added drop by drop till half its bulk is gone, when No. 1 is added to the remainder, and the double solution is dropped in as before. When all is added there ought to be formed an emulsion daylight. The flask containing the emulsion is next placed in which is very ruddy when examined by gaslight, or orange by boiling water, which is kept in a state of ebullition for about threequarters of an hour. It is then ready, when the contents of the flask have cooled down to about 100° F., for the addition of No. 5, and finally be dissolved. The gelatin emulsion thus formed is which should in the interval be placed in 2 oz. of water to swell placed in a cool place to set, after which it is turned into a piece of coarse canvas or mosquito netting made into a bag. By squeezing, threads of gelatin containing the sensitive salt can be made to fall into cold water; by this means the soluble salts are extracted. This is readily done in two or three hours by frequently changing the water, or by allowing running water to flow over the emulsion-threads. The gelatin is next drained by straining canvas over a jar and turning out the threads on to it, after which it is placed in a flask, and warmed till it dissolves, half an ounce of alcohol being added. Finally it is filtered through chamois leather or swansdown calico. In this state it is ready for the plates. The other method of forming the emulsion is with ammonia. The same quantities as before are weighed out, but the solutions of Nos. 2 and 3 are first mixed together and No. 4 is dissolved in 1 oz. of water, and strong ammonia of specific gravity 880 added to it till the oxide first precipitated is just redissolved. This solution is then dropped into Nos. 2 and 3 as previously described, and finally No. 1 is added. In this case no boiling is required; but to secure rapidity it is as well that the emulsion should be kept an hour at a temperature of about 90° F., after which half the total 1 For further details the reader is referred to Instruction in Photo- quantity of No. 5 is added. When set the emulsion is washed, graphy, 11th ed., p. 362. drained, and redissolved as before; but in order to give tenacity

to the gelatin the remainder of No. 5 is added before the addition of the alcohol, and before filtering.

Coating the Plates.-Glass plates are best cleaned with nitric acid, rinsed, and then treated with potash solution, rinsed again, and dried with a clean cloth. They are then ready for receiving the emulsion, which, after being warmed to about 120° F., is poured on them to cover well the surface. This being done, the plates are placed on a level shelf and allowed to stay there till the gelatin is thoroughly set; they are then put in a drying cupboard, through which a current of warm air is made to pass. It should be remarked that the warmth is only necessary to enable the air to take up the moisture from the plates. They ought to dry in about twelve hours, and they are ready for use. Exposure. With a good emulsion and on a bright day the exposure of a plate to a landscape, with a lens whose aperture is one-sixteenth that of the focal distance, should not be more than one-half to one-fifth of a second. This time depends, of course, on the nature of the view; if there be foliage in the immediate foreground it will be longer. In the portrait-studio, under the same circumstances, an exposure with a portrait lens may be from half a second to four or five seconds.

Development of the Plate.-To develop the image either a ferrous oxalate solution or alkaline pyrogallic acid may be used. No chemical restrainer such as potassium bromide is necessary, since the gelatin itself acts as a physical restrainer. If the alkaline developer be used, the following may be taken as a good standard:Pyrogallol

Water

I.

Citric acid

2.

Potassium bromide

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3.

Water

.

50 grs. ΙΟ

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I OZ.

10 grs.

I oz.

-

Toning and Fixing the Print.-The next operation is to tone and fix the print. In the earlier days this was accomplished by means of a bath of sel d'or-a mixture of hyposulphite of soda and gold chloride. This gilded the darkened parts of the print which light had reduced to the semi-metallic state: and on the removal of the chloride by means of hyposulphite an image composed of metallic silver, an organic salt of silver and gold was left behind. There was a suspicion, however, that part of the coloration was due to a combination of sulphur with the silver, not that pure silver sulphide is in any degree fugitive, but the sulphuretted organic salt of silver seems to be liable to change. This gave place to a method of alkaline toning, or rather, we should say, of neutral toning, by employing gold chloride with a salt, such as the carbonate or acetate of soda, chloride of lime, borax, &c. By this means there was no danger of sulphurization during the toning, to which the method by sel d'or was prone owing to the decomposition of the hyposulphite. The substances which can be employed in toning seem to be those in which an alkaline base is combined with a weak acid, the latter being readily displaced by a stronger acid, such as nitric acid, which must exist in the paper after printing. This branch of photography owes much to the Rev. T. F. Hardwich, he having carried on extensive researches in connexion with it during 1854 and subsequent years. A. Davanne and A. Girard, a little later, also investigated the matter with fruitful results.

The following may be taken as two typical toning-baths:-
Gold chloride

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I part. Io parts. 5000 100 31 4000

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I part. 4000 parts

I dr. 9

One dram of each of these is taken and the mixture made up to 2 oz. with water. The plate is placed in a dish and the above poured over it without stoppage, whereupon the image gradually appears and, if the exposure has been properly timed, gains sufficient density for printing purposes. It is fixed in a solution of hyposulphite of soda, as in the other processes already described, and then thoroughly washed for two or three hours to eliminate all the soluble salt. This long washing is necessary on account of the nature of the gelatin.

Intensifying the Negative. Sometimes it is necessary to intensify the negative, which can be done in a variety of ways with mercury salts. An excellent plan, introduced by Chapman Jones, is to use a saturated solution of mercuric chloride in water. After thorough washing the negative is treated with ferrous oxalate. This process can be repeated till sufficient density is attained. With most other methods with mercury the image is apt to become yellow and to fade; with this apparently it is not. Varnishing the Negative.-The negative is often protected by receiving first a film of plain collodion and then a coat of shellac or other photographic varnish. This protects the gelatin from moisture and also from becoming stained with the silver nitrate owing to contact with the sensitive paper used in silver printing. Another varnish is a solution of celloidin in amyl acetate. This is an excellent protection against damp.

Printing Processes.

The first printing process may be said to be that of Fox Talbot (see above), which has continued to be generally employed (with the addition of albumen to give a surface to the print-an addition first made, we believe, by Fox Talbot).

Paper for printing is prepared by mixing 150 parts of ammonium chloride with 240 parts of spirits of wine and 2000 parts of water, though the proportions may vary. These ingredients are dissolved, and the whites of fifteen fairly-sized eggs are added and the whole beaten up to a froth. In hot weather it is advisable to add a drop of carbolic acid to prevent decomposition. The albumen is allowed two or three days to settle, when it is filtered through a sponge placed in a funnel, or through two or three thicknesses of fine muslin, and transferred to a flat dish. The paper is cut of convenient size and allowed to float on the solution for about a minute, when it is taken off and dried in a warm room. For dead prints, on which colouring is to take place, plain salted paper is useful. It can be made of the following proportions-90 parts of ammonium chloride, 100 parts of sodium citrate, 10 parts of gelatin, 5000 parts of distilled water. The gelatin is first dissolved in hot water and the remaining components are added. It is next filtered, and the paper allowed to float on it for three minutes, then withdrawn and dried.

Sensitizing Bath. To sensitize the paper it is floated on a 10% solution of silver nitrate for three minutes. It is then hung up and allowed to dry, after which it is ready for use. To print the image the paper is placed in a printing frame over a negative and exposed to light. It is allowed to print till such time as the image appears rather darker than it should finally appear.

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These are mixed together, the water being warmed." When cool the solution is ready for use. In toning prints there is a distinct difference in the modus operandi according to the toning-bath employed. Thus in the first two baths the print must be thoroughly washed in water to remove all free silver nitrate, that salt forming no part in the chemical reactions. On the other hand, where free chlorine is used, the presence of free silver nitrate or some active chlorine absorbent is a necessity. In 1872 Abney showed that with such a toning-bath free silver nitrate might be eliminated. and if the print were immersed in a solution of a salt such as lead nitrate the toning action proceeded rapidly and without causing any fading of the image whilst toning, which was not the case when the free silver nitrate was totally removed and no other chlorine absorbent substituted. This was an important factor, and one which had been overlooked. In the third bath the free silver nitrate should only be partially removed by washing. The print, having been partially washed or thoroughly washed, as the case may be, is immersed in the toning-bath till the image attains a purple or bluish tone, after which it is ready for fixing. The solution used for this purpose is a 20% solution of hyposulphite of soda, to which it is best to add a dew drops of ammonia in order to render it alkaline. About ten minutes suffice to effect the conversion of the chloride into hyposulphite of silver, which is soluble in hyposulphite of soda and can be removed by washing. The organic salts of silver seem, however, to form a different salt, which is partially insoluble, but which the ammonia helps to remove. If it is not removed there is a sulphur compound left behind, according to J. Spiller, which by time and exposure becomes yellow. The use of potassium cyanide for fixing prints is to be avoided, as this reagent attacks the organic coloured oxide which, if removed, would render the print a ghost. The washing of silver prints should be very complete, since it is said that the least trace of hyposulphite left behind renders the fading of the image a mere matter of time. The stability of a print has been supposed to be increased by immersing it, after washing, in a solution of alum. The alum. like any acid body, decomposes the hyposulphite into sulphur and sulphurous acid. If this be the case, it seems probable that the destruction of the hyposulphite by time is not the occasion of fading, but that its hygroscopic character is. This, however, is a moot point. It is usual to wash the prints some hours in running water. We have found that half a dozen changes of water, and between successive changes the application of a sponge to the back of each print separately, are equally or more efficacious. On drying the print assumes a darker tone than it has after leaving the fixing bath.

Different tones can thus be given to a print by different toningbaths; and the gold itself may be deposited in a ruddy form or in a blue form. The former molecular condition gives the red and sepia tones, and the latter the blue and black tones. The degree of minute subdivision of the gold may be conceived when it is

PHOTOGRAPHY

493

stated that, on a couple of sheets of albuminized paper fully printed, | washing it in water, when a white picture on an orange ground was the gold necessary to give a decided tone does not exceed half a grain.

Collodio-chloride Silver Printing Process.-In the history of the emulsion processes we stated that Gaudin attempted to use silver chloride suspended in collodion, but it was not till the year 1864 that any practical use was made of the suggestion so far as silver printing is concerned. In the autumn of that year George Wharton Simpson worked out a method which has been more or less successfully employed. The formula appended is Simpson's:

I.

2.

Silver nitrate

Distilled water

Strontium chloride.
Alcohol
Citric acid

3. Alcohol

60 parts.

60

64 1000

64 1000

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To every 1000 parts of plain collodion 30 parts of No. 1, previously mixed with 60 parts of alcohol, are added; 60 parts of No. 2 are next mixed with the collodion, and finally 30 parts of No. 3. This forms an emulsion of silver chloride and also contains citric acid and silver nitrate. The defect of this emulsion is that it contains a large proportion of soluble salts, which are apt to crystallize out on drying, more particularly if it be applied to glass plates. The addition of the citric acid and the excess of silver nitrate is the key to the whole process; for, unless some body were present which on exposure to light was capable of forming a highly-coloured organic oxide of silver, no vigour would be obtained in printing; If pure chloride be used, though an apparently strong image would be obtained, yet on fixing only a feeble trace of it would be left, and the print would be worthless. The collodio-chloride emulsion may be applied to glass, or to paper, and the printing carried on in the usual manner. of lime or by ammonium sulphocyanide and gold, which is practiThe toning takes place by means of the chloride cally a return to the sel d'or bath. this procedure does not seem so prone to be decomposed by keeping The organic salt formed in as does that formed by albumen, and the washing can be more completely carried out. which are collodio-chloride. There are in the market several papers Gelatino-citro-chloride Emulsion.-A modified emulsion printing process was introduced by Abney in 1881, which consisted in suspending silver chloride and silver citrate in gelatin, there being no excess of silver present. The formula of producing it is as follows:Sodium chloride

I.

2.

Potassium citrate
Water

Silver nitrate
Water

Gelatin

3. Water

40 parts.
40

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300 1700 ""

Nos. 2 and 3 are mixed together whilst warm, and No. I is then gently added, the gelatin solution being kept in brisk agitation. This produces the emulsion of citrate and chloride of silver. The gelatin containing the suspended salts is heated for five minutes at boiling point, when it is allowed to cool and subsequently slightly washed, as in the gelatino-bromide emulsion. It is then ready for application to paper or glass. The prints are of a beautiful colour, and seem to be fairly permanent. They may be readily toned by the borax or by the chloride of lime toning-bath, and are fixed with the hyposulphite solution of the strength before given. Most, if not all, of the gelatin papers now extant are made somewhat after

this manner.

Printing with Salts of Uranium.—The sensitiveness of the salts of uranium to light seems to have been discovered by Niepce, and was subsequently applied to photography by J. E. Burnett in England. One of the original formulae consisted of 20 parts of uranic nitrate with 600 parts of water. Paper, which is better if slightly sized previously with gelatin, is floated on this solution. When dry it is exposed beneath a negative, and a very faint image is produced; but it can be developed into a strong one by 6 to 10% solution of silver nitrate to which a trace of acetic acid has been added, or by a 2% solution of gold chloride. In both these cases the silver and gold are deposited in the metallic state. developer is a 2% solution of potassium ferrocyanide to which a Another trace of nitric acid has been added, sufficient to give a red coloration. The development takes place most readily by letting the paper float on these solutions.

Self-toning Papers.-There are several self-toning papers based on the chloride emulsion process. amount of gold to tone the print. The print is produced in the These contain the necessary ordinary way and then immersed in salt and water or in some cases potassium sulphocyanide. The print is finished by immersing in weak hyposulphite of soda.

Printing with Chromates: Carbon Prints.-The first mention of the use of potassium bichromate for printing purposes seems to have been made by Mungo Ponton in May 1839, when he stated that paper, if saturated with this salt and dried, and then exposed to the Sun's rays through a drawing, would produce a yellow picture On an orange ground, nothing more being required to fix it than

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obtained. In 1840 Edmond Becquerel announced that paper
Massachusetts, in the following year, produced copies of bank-notes
sized with iodide of starch and soaked in potassium bichromate was,
by using gum arabic with potassium bichromate spread upon a
on drying, more sensitive than unsized paper Joseph Dixon of
lithographic stone, and, after exposure of the sensitive surface
through a bank-note, by washing away the unaltered gum and
inking the stone as in ordinary lithography. The same process,
with slight modifications, has been used by Simonau and Toovey
of Brussels, and produces excellent results.
however, was published in the Scientific American for 1854, and
Dixon's method,
consequently, as regards priority, it ranks after Fox Talbot's photo-
engraving process (see below), published in 1852. On the 13th
of December 1855 Alphonse Poitevin took out a patent in England,
potassium bichromate. This idea was taken up by John Pouncey
in which he vaguely described a method of taking a direct carbon-
print by rendering gelatin insoluble through the action of light on
carbon-prints, notwithstanding that Testud de Beauregard took
of Dorchester, who perhaps was the first to produce veritable
as described by him, it was by no means in a perfect state, half-
out a somewhat similar patent to Poitevin's at the end of 1857.
tones being wanting. The cause of this was first pointed out by
Pouncey published his process on the 1st of January 1859; but,
tive film, however thin it may be, two distinct surfaces must be
Abbé Laborde in 1858, whilst describing a kindred process in a
recognized-an outer, and an inner which is in contact with the
note to the French Photographic Society. He says,
"In the sensi-
washing, therefore, the half-tones lose their hold on the paper and
paper. The action of light commences on the outer surface; in the
are washed away."
requisites.
Photographic Society of London he says,
to get rid of this defect in carbon printing. In a paper to the
J. C. Burnett in 1858 was the first to endeavour
placed in contact with the negative in the pressure-frame, as it
unprepared side (and not its prepared side, as in ordinary printing)
(2) that in printing the paper should have its
There are two essential
is only by printing in this way that we can expect to be able after-
pared side the attainment of half-tones by washing away more or
wards to remove by washing the unacted-upon portions of the
mixture. In a positive of this sort printed from the front or pre-
less depth of the mixture, according to the depth to which it has
been hardened, is prevented by the insoluble parts being on the
surface and in consequence protecting the soluble part from the
action of the water used in washing;
removed, or by steeping very long till the inner soluble part is
so that either nothing is
sufficiently softened the whole depth comes bodily away, leaving
the paper white." This method of exposing through the back of
the paper was crude and unsatisfactory, and in 1860 Fargier
patented a process in which, after exposure to light of the gelatin
paper support and could be transferred to glass. Poitevin success-
film which contained pigment, the surface was coated with collodion,
fully opposed this patent, for he had used this means of detaching
and the print placed in warm water, where it separated from the
the films in his powder-carbon process, in which ferric chloride and
carbon-printing, and J. W. Swan took up the matter, and in 1864
secured a patent. One of the great features in Swan's innovations
tartaric acid were used. Fargier at any rate gave an impetus to
made by coating paper with a mixture of gelatin, sugar and
colouring matter, and rendered sensitive to light by means of
was the production of what is now known as
potassium or ammonium bichromate.
carbon-tissue,'
Swan placed the printed carbon-tissue on an india-rubber surface,
in hot water, the paper backing stripped off, and the soluble gelatin
After exposure to light
containing colouring matter washed away.
to which it was made to adhere by pressure. The print was immersed
then be retransferred to its final support of paper. In 1869 J. R.
Johnson of London took out a patent in which he claimed that
carbon-tissue which had been soaked in water for a short period,
The picture could
by its tendency to swell further, would adhere to any waterproof
surface such as glass, metal, waxed paper, &c., without any adhesive
material being applied. This was a most important improvement.
Johnson also applied soap to the gelatin to prevent its excessive
brittleness on drying, and made its final support of gelatinized
paper, rendered insoluble by chrome alum. In 1874 J. R. Sawyer
coated with gelatin and treated with an ammoniacal solution of
shellac in borax, on which wax or resin was rubbed. The advantage
patented a flexible support for developing on; this was a sized paper
tendency to contract from the lighter parts, which they were apt
of this flexible support is that the dark parts of the picture have no
original process. With this patent, and minor improvements
to do when a metal plate was used, as was the case in Johnson's
made since, carbon-printing has arrived at its present state of
perfection.

kind), 15 grains of soap, 21 grains of sugar and from 4 to 8 grains
According to P. E. Liesegang, the carbon-tissue when prepared
of dry colouring matter. The last-named may be of various kinds,
on a large scale consists of from 120 to 150 grains of gelatin (a soft
from lamp-black pigment to soluble colours such as alizarin. The
gelatin, sugar and soap are put in water and allowed to stand for
an hour, and then melted, the liquid afterwards receiving the

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the whole became coagulated rendered these unmanageable. It
at last occurred to him that if the hardening action of light were
utilized by exposing the surface next the plate to light after or
before exposing the front surface to the film and the image, the
necessary hardness might be given to the gelatin without adding
any chemical hardeners to it. In Tessie de Motay's process the
hardening was almost absent, and the plates were consequently not
durable. It is evident that to effect this one of two things had to
be done: either the metallic plate used by Tessie de Motay must
be abandoned, or else the film must be stripped off the plate and
exposed in that manner. Albert adopted the transparent plate,
and his success was assured, since instead of less than a hundred
impressions being pulled from one plate he was able to take over a
thousand. This occurred about 1867, but the formula was not
published for two or three years afterwards, when it was divulged
by Ohm and Grossman, one of whom had been employed by Albert
of Munich, and had endeavoured to introduce a process which
resembled Albert's earlier efforts. The name of "Lichtdruck" was
given about this time to these surface-printing processes, and Albert
may be considered, if not the inventor, at all events the perfecter of
the method. Another modification of “Lichtdruck was patented
in England by Ernest Edwards under the name of "heliotype."
Woodbury Type.-This process was invented by W. Woodbury
about the year 1864, though we believe that J. W. Swan had been
working independently in the same direction about the same time.
In October 1864 a description of the invention was given in the
Photographic News. Marc Antoine A. Gaudin claimed the principle
of the process, insisting that it was old, and basing his pretensions
on the fact that he had printed with translucent ink from intaglio
blocks engraved by hand; but at the same time he remarked that
the application of the principle might lead to important results.
It was just these results which Woodbury obtained, and for which
he was entitled to the fullest credit. Woodbury subsequently
introduced certain modifications, the outcome being what is known
as the "stannotype process," of which in 1880 he read a description
before the French Photographic Society (see PROCESS).
Photo-lithography.-Reference has been made to the effect of
light on gelatin impregnated with potassium bichromate, whereby
the gelatin becomes insoluble, and also incapable of absorbing water
where the action of the light has had full play. It is this last
phenomenon which occupies such an important place in photo-
lithography. In the spring of 1859 E. J. Asser of Amsterdam
produced photographs on a paper basis in printer's ink. Being
anxious to produce copies of such prints mechanically, he conceived
the idea of transferring the greasy ink impression to stone, and
multiplying the impressions by mechanical lithography. Following
very closely upon Asser, J. W. Osborne of Melbourne made a
similar application; his process is described by himself in the
Photographic Journal for April 1860 as follows: "A negative is
produced in the usual way, bearing to the original the desired ratio.
A positive is printed from this negative upon a sheet of
(gelatinized) paper, so prepared that the image can be transferred to
stone, it having been previously covered with greasy printer's ink.
The impression is developed by washing away the soluble matter
with hot water, which leaves the ink on the lines of print of the
map or engraving." The process of transferring is accomplished in
the ordinary way. Early in 1860 Colonel Sir H. James, RE,
F.R.S., brought forward the Southampton method of photo-litho

colours, which have been ground on a slab. The mixture is filtered through fine muslin. In making the tissue in large quantities the two ends of a piece of roll-paper are pasted together and the paper hung on two rollers; one of wood about 5 in. in diameter is fixed near the top of the room and the other over a trough containing the gelatin solution, the paper being brought into contact with the surface of the gelatin by being made to revolve on the rollers. The thickness of the coating is proportional to the rate at which the paper is drawn over the gelatin: the slower the movement, the thicker the coating. The paper is taken off the rollers, cut through, and hung up to dry on wooden laths. If it be required to make the tissue sensitive at once, 120 grains of potassium bichromate should be mixed with the ingredients in the above formula. The carbon-tissue when prepared should be floated on a sensitizing bath consisting of one part of potassium bichromate in 40 parts of water. This is effected by turning up about 1 in. froin the end of the sheet of tissue (cut to the proper size), making a roll of it, and letting it unroll along the surface of the sensitizing solution, where it is allowed to remain till the gelatin film feels soft. It is then taken off and hung up to dry in a dark room through which a current of dry warm air is passing. Tissue dried quickly, though not so sensitive, is more manageable to work than if more slowly dried. As the tissue is coloured, it is not possible to ascertain by inspection whether the printing operation is sufficiently carried out, and in order to ascertain this it is usual to place a piece of ordinary silvered paper in an actinometer, or photometer, alongside the carbon-tissue to ascertain the amount of light that has acted on it. There are several devices for ascertaining this amount, the simplest being an arrangement of a varying number of thicknesses of gold-beater's skin. The value of 1, 2, 3, &c., thicknesses of the skin as a screen to the light is ascertained by experiment. Supposing it is judged that a sheet of tissue under some one negative ought to be exposed to light corresponding to a given number of thicknesses, chloride of silver paper is placed alongside the negative beneath the actinometer and allowed to remain there until it takes a visible tint beneath a number of thicknesses equivalent to the strength of the negative. After the tissue is removed from the printing-frame-supposing a double transfer is to be made it is placed in a dish of cold water, face downwards, along with a piece of Sawyer's flexible support. When the edges of the tissue begin to curl up, its surface and that of the flexible support are brought together and placed flat. The water is pressed out with an indiarubber squeezer or squeegee " and the two surfaces adhere. About a couple of minutes later they are placed in warm water of about 90° to 100° F., and the paper of the tissue, loosened by the gelatin solution next it becoming soluble, can be stripped off, leaving the image (reversed as regards right and left) on the flexible support. An application of warm water removes the rest of the soluble gelatin and pigment. When dried the image is transferred to its permanent support. This usually consists of white paper coated with gelatin and made insoluble with chrome alum, though it may be mixed with barium sulphate or other similar pigments. This transfer-paper is made to receive the image by being soaked in hot water till it becomes slimy to the touch; and the surface of the damped print is brought into contact with the surface of the retransfer-paper, in the same manner as was done with the flexible support and the carbon-tissue. When dry the retransfer-paper bearing the gelatin image can be stripped off the flexible support, which may be used again as a temporary support for other pictures.graphy, which had been carefully worked out by Captain de Courcy If a reversed negative be used the image may be transferred at once to its final support instead of to the temporary flexible support, which is a point of practical value, since single-transfer are better than double-transfer prints.

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Printing with Salts of Iron.-Sir John Herschel and Robert Hunt entered into various methods of printing with salts of iron. At the present time two or three are practised, being used in draughtsmen's offices for copying tracings (see SUN-COPYING),

Photo-mechanical Printing Processes.-Poitevin claimed to have discovered that a film of gelatin impregnated with potassium bichromate, after being acted upon by light and damping, would receive greasy ink on those parts which had been affected by light. But Paul Oreloth seems to have made the discovery previous to 1854, for in his patent of that year he states that his designs were inked with printing ink before being transferred to stone or zinc. C. M. Tessie de Motay (in 1865) and C. R. Marechal of Metz, however, seem to have been the first to produce half-tones from gelatin films by means of greasy ink. Their general procedure consisted in coating metallic plates with gelatin impregnated with potassium or ammonium bichromate or tri-chromate and mercuric chloride, then treating with silver oleate, exposing to light through a negative, washing, inking with a lithographic roller, and printing from the plates as for an ordinary lithograph. The half-tints by this process were very good, and illustrations executed by it are to be found in several existing works. The method of producing the plates, however, was most laborious, and it was simplified by A. Albert of Munich. He had been experimenting for many years, endeavouring to make the gelatin films more durable than those of Tessie de Motay. He added gum-resins, alum, tannin and other such matters, which had the property of hardening gelatin; but the difficulty of adding sufficient to the mass in its liquid state before

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Scott, R.E. The " papyrotype process was published by Abney in 1870 (see LITHOGRAPHY and PROCESS).

Photographs in Natural Colours.

The first notice on record of coloured light impressing its own colours on a sensitive surface is in the passage already quoted from the Farbenlehre of Goethe, where T. J. Seebeck of Jena (1810) describes the impression he obtained on paper impregnated with moist silver chloride. In 1839 Sir J. Herschel (Athenaeum, No. 621) gave a somewhat similar description. In 1848 Edmond Becquerel succeeded in reproducing upon a daguerreotype plate not only the colours of the spectrum but also, up to a certain point, the colours of drawings and objects. His method of proceeding was to give the silver plate a thin coating of silver chloride by immersing it in ferric or cupric chlorides. It may also be immersed in chlorine water till it takes a feeble rose tint. Becquerel preferred to chlorinize the plate by immersion in a solution of hydrochloric acid in water, attaching it to the positive pole of a voltaic couple. whilst the other pole he attached to a platinum plate also immersed in the acid solution. After a minute's subjection the current the plate took successively a grey, a yellow, a violet and a blue tint, which order was again repeated. When the violet tint appeared for the second time the plate was withdrawn and washed and dried over a spirit-lamp. In this state it

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