« السابقةمتابعة »
development is conducted in a flat dish, sometimes the solution is poured on the plate.1 The unreduced salts are eliminated by either cyanide of potassium or sodium posul hite. Intensity may be given to the image, if requisite, ext er be ore or after the “ fixing " operation. Where resort is had to ferrous oxalate development, the developer is made in one of two wa s—(l) b saturating a saturated solution of neutral potassium oxa ate with errous oxalate, and adding an equal volume of a solution (10 grains to 1 oz. of water) of potassium bromide to restrain the action, or (2) by mixin , according to Eder's plan, 3 volumes by measure of a saturate 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 Pracess.——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 187 3 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 precipitatinga 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 branchingofi 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 r881 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 alc'ohol 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.
1 For further details the reader is referred to Instrudion in Photography, llth ed., p. 362.
up in I oz. of water, and is then dissolve
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 desired sensibility only a few hours were necessary. A further advance was made by boiling the emulsion, first practised, we 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.
TECHNIQUE or PHOTOGRAPHY
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 extreme ease with which the equilibrium of the molecules is upset in giving rise to the molecule which is developable. 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 daylight be employed, it is as well to have a double thickness of orange paper. The following should be weighed out :—
1. Potassium iodide . . . 5 grs. 2. Potassium bromide . . . . . . 135 ,, 3. Nelson’s No. i photographic gelatin 30 ,, 4. Silver nitrate . . . . . . . r75 ,,
Autotype or other hard gelatin . 100 ,, 5' Nelsons No. ! gelatin . . . . loo ,,
Nos. 3 and 5 are rapidly covered with water or washed for a few seconds under the ta to get rid of an dust. No. 2 is dissolved in IQ oz. of water, an a little tincture o iodine added till it assumes a light sherry colour. No. I is dissolved in 60 minims of water. No. 4 is dissole in 5 oz. of water, and No. iis allowed to swell
d by eat. All the flasks containing these solutions are placed in water at 150° F. and carried into the ‘ dark room," as the oran e-lighted chamber is ordinarily called; Nos. 3 and 4 are then mix together in a jar or flask, and No. 2 added drop by drop till half its bulk is gone, when No. l is added to the remainder, and the double solution is dropped in as before. \Vhen all is added there ought to be formed an emulsion which is ve ruddy when examined by gaslight, or orange by daylight. ‘T e flask containing the emulsion is next placed in boiling water, which is kept in a state of ebullition for about three
uarters of an hour. It is then read ', when the contents of the
ask have cooled down to about 100° ., for the addition of No. 5, which should in the interval be placed in 2 oz. of water to swell and finally be dissolved. The gelatin emulsion thus formed is placed in a. cool place to set, after which it is turned into a piece of coarse canvas or mo uito 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 turnin out the threads on to it, after which it is placed in a. flask, an 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 ravity ~880 added to it till the oxide first precipitated is Just re issolved. This solution is then dropped into Nos. 2 and 3 as previously described, and finally No. I 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 tem rature of about ° F., after which half the total
uantity of NE: 5 is added. en set the emulsion is washed, 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 filterin .
Coating the Plates.—Glass pates 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 on] necessary to enable the air to take up the moisture from the p ates. They ought to dry in about twelve hours, and the are ready for use.
ExPosure.— ith 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.
Developmmt 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 elatin itself acts as a physical restrainer. If the alkaline deve oper be used, the following may be taken as a good standard :—
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 graduall appears and, if the exposure has been properly timed, gains su ~ cient density for printing purposes. It is fixed in a solution of hyPOsulphite of soda, as in the other processes already described, and then thoroughl 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 b Chapman Jones, is to use a saturated solution of mercuric chlori e 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 mercu the image is apt to become yellow and to fade; with this a parent y it is not.
Vanishing the egattve.—The negative is often protected by receiving first a film of plain collodion and then a coat of shellac or other photo raphic varnish. This protects the gelatin from moisture and a so from becoming stained with the silver nitrate owin to contact with the sensitive paper used in silver printing. Anot er varnish is a solution of cellOidin in amyl acetate. This is an excellent protection against damp.
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).
Pa r 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 u 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 throu h a sponge placed in a funne ,or through two or three thicknesseso fine muslin, and transferred to a flat dish. The pa r is cut of convenient size and allowed to float on the solution or about a minute, when it is taken off and dried in a warm room. For dead prints, on which colourin is to take place, plain salted paper is useful. It can be made 0 the following proportions—90 rts of ammonium chloride,‘ 100 parts of sodium citrate, 10 parts 0 gelatin, 000 parts of distilled water. The gelatin is first dissolved in ct water and the remainin components are added. It is next filtered, and the paper allowe 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 rint till such time as the image appears rather darker than it shou d finally appear.
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 so! d'or—a mixture of hyposul hite of soda and gold chloride. This gilded the darkened parts 0 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 0 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 gaveplace to a method of alkaline toning, or rather, we should say, of neutral toning, by employin gold chloride with a salt, such as the carbonate or acetate 0 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 tonin seem to be those in which an alkaline base is combined with a wea 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.
Gold chloride . . . . . . . . 2 parts. Chloride of lime . . . . . . . . 2 ,, Chalk . . . 40 :1"
These are mixed to ether, the water being warmed. When cool the solution is ready or use. In toning rints there is a distinct difference in the modus operandi accordin 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 si ver nitrate might be eliminated. and if the print were immersed in a solution of a salt such as lead nitrate the toning action proceeded rapidl and without causing any fading of the image whilst toning, w ich was not the case when the free silver nitrate was totally removed and no other chlorine absorbent substituted. This was an im rtant factor. and one which had been overlooked. In the thir 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 til 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 by ulphite 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 difi'erent 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 . Spiller, which b time and exposure becomes yellow.
The use 0 potassium cyani e for fixing prints is to be avoided. as this rea ent attacks the organic coloured oxide which, if removed. would render the print a ghost. The washing of silver rints should be very complete, since it is said that the least trace of yposulphite left behind renders the fading of the image a mere matter of timev 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 moo: point. It is usual to wash the prints some hours in runni water. We have found that half a dozen changes of water, a 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 t an it has after lmving the fixing bath.
Different tones can thus be given to a print by difierent tuningbaths; and the gold itself may be deposited in a ruddy form or is a blue form. The former molecular condition gives the red and sepia tones, and the latter the blue and black tones. The (kin-e of minute subdivision of the gold may be conceived when it is
rinting is Concerned. In the autumn of that year George Wharton im son’ worked out a method which has been more or less sucully employed. The formula appended is Simpson's:— I Silver nitrate . . . . . 60 parts. ' Distilled water . 60 ,, Strontium chloride . . 64 ,, 2' Alcohol . . 1000 ,, Citric acid . 64 ,, 3- Alcohol . 1000 ,,
To every 1000 parts of plain collodion 30 parts of No. I, reviously mixed with 60 parts of alcohol, are added; 60 parts of o. 2 are next mixed with the collodion, and finally 30 parts of No. 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 rocess; for, unless some body were present which on ex sure to light was capable of forming a hi hly-coloured organic oxi e of silver, no vigour would be obtaine in printing. If pure chloride be used, thou h an ap arently strong ima e would be obtained, yet on fixing on a feeb e trace of it woul be left, and the print would be worthless. The collodio-chloride emulsion may be applied to glass, or to pa r, and the printing carried on in the usual manner. The toning ta es place by means of the chloride of lime or by ammonium sulphocyanlde and gold, which is practically a return to the sel d'ar bath. The organic salt formed in this procedure does not seem so prone to be decom sed by keeping as does that formed by albumen, and the washing can be more completely carried out. There are in the market several papers which are collodio-chloride.
Gelatino-citro-chloride Emulsion—A modified emulsion rinting process was introduced by Abney in l88l, which consist in suspending silver chloride and silver citrate in elatin, there being no excess of silver present. The formula of pro ucing it is as follows :—
Sodium chloride 40 parts. 1. Potassium citrate 40 ,, Water . . 500 ,, 2 Silver nitrate 150 ,, ' 'Water 500 ,, Gelatin 300 ,, Water 1700 0
Nos. 2 and 3 are mixed to ether whilst warm, and No. I is then ently added, the gelatin so ution being kept in brisk agitation. his produces the emulsion of citrate and chloride of silver. The elatin containing the suspended salts is heated for five minutes at
goiling point, when it is allowed to cool and subsequently slightly washed, as in the gelatino-bromide emulsion. It is then ready for application to pa r or glass. The grins are of a beautiful colour, and seem to be airly permanent. hey may be readily toned by the borax or by the chloride of lime toning-bath, and are fixed wit the h posul hlte solution of the strength before given. Most, if not a l, of tfi>e 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 b Niepce, and
was subsequently a plied to photography b J. h. Burnett in England. One of tiie original formulae consisted of 20 rts of uranic nitrate with 600 parts of water. Paper, which is tter if
sli htly sized previously with gelatin, is floated on this solution. VV en dry it is exposed beneath a negative, and a very faint image is produced; but lt can be develo d into a strong one by 6 to 10% solution of silver nitrate to whiche 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. Another developer is a 2% solution of potassium ferrocyanidc to which a t race 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.
Sel darting Papers—There are several self-toning papers based on t e chloride emulsion process. These contain the necessary amount of gold to tone the print. The print is produced in the ordinary way and then immersed in salt and water or in some cases potassium sulphocyanide. The print is finished by immerslng ln \vcak hyposulphite of soda.
Printing with Chromates: Carbon Prints—The first mention of the use of potassium bichromate for rinting purposes seems to have been made by Mungo Ponton in ay 1839, when he stated that paper, if saturated With this salt and dried, and then ex osed to the sun‘s rays through a drawing, would produce a ye ow icture )2: an orange ground, nothing more being required to fix it than
washing it in water, when a white picture on an oran e ground was obtained. In ' I8 0 Edmond Bee uerel announce that paper sized with iodide o starch and soake in potassium bichromate was, on drying, more sensitive than unsized paper Joseph Dixon of Massachusetts, in the following year, produced copies of bank-notes by using gum arabic with potassium bichromate spread upon a 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 lithogra hy. The same rocess, with slight modifications, has been used y Simonau and oove of Brussels, and produces excellent results. Dixon's meth , however, was published in the Scientific American for 1854, and consequently, as regards priority, it ranks after Fox Talbot's photoengraving process (see below), published in 1852. On the 13th of December 1855 Alphonse Poitevin took out a patent in England, in which he vaguely described a method of taking a direct carbonprint by rendering gelatin insoluble through the action of light on potassium bichromate. This idea was taken up by aJohn Pouncey of Dorchester, who rhaps was the first to pro uce veritable carbon-prints, notwit standing that Testud de Beauregard took out a somewhat similar patent to Poitevin’s at the end of 1857.
Pounceygpublished his process on the Ist of January 1859; but, as describ by him, it was by no means in a perfect state, halftones being wanting. The cause of this was first pointed out by Abbé Laborde in 1858, whilst describing a kindred recess in a note to the French Photographic Society. He says, “ n the sensitive film, however thin it may be, two distinct surfaces must be recognized—an outer, and an inner which is in contact with the paper. The action of li ht commences on the outer surface; in the washing, therefore, the alf-tones lose their hold on the paper and are washed away." J. C. Burnett in 1858 was the first to endeavour to get rid of t is defect in carbon printin . In a paper to the Photographic Society of London he says, “ here are two essential requisites. . . (2) that in printing the paper should have its unpre ared side (and not its prepared side, as in ordinary printing) place in contact with the negative in the pressure-frame, as it is only by printing in this way that we can expect to be able afterwards to remove by washing the unacted-upon rtions of the mixture. In a positive of this sort printed from t e front or prepared side the attainment of half-tones by washing away more or ess depth of the mixture, according to the depth to which it has been hardened, is prevented by the insoluble parts bein on the surface and in consequence protecting the soluble art mm the action of the water used in washing; so that eit er nothing is removed, or by steeping very long till the inner soluble art is sufficiently softened the whole depth comes bodily away, eavin the paper white." This method of exposing through the back 0 the paper was crude and unsatisfactory, and in X860 Fargier patented a process in which, after exposure to light of the latin film which contained pigment, the surface was coated with col odion, and the print placed in warm water, where it separated from the paper support and could be transferred to glass. Poitevin successul y opposed this patent, for he had used this means of detachin the films in his powder-carbon process, in which ferric chloride an tartaric acid were used. Fargler at any rate gave an impetus to 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 was the production of what is now known as “carbon-tissue," made by coating pa r with a mixture of elatin, sugar and colouring matter, angerendered sensitive to fight by means of
otassium or ammonium bichromate. After exposure to light
wan placed the printed carbon-tissue on an india-rubber surface, to which it was made to adhere by pressure. The print was immersed in hot water, the paper backing stripped ofi‘, and the soluble gelatin containing colouring matter washed awa . The icture could then be retransferred to its final support 0 paper. Yn 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, 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 app ied soap to the gelatin to prevent its excessive
rittleness on dryin , and made its final support of elatinized paper, rendered insoguble b chrome alum. In 1874 J. . Sawyer patented a flexible support or developing on; this was a sized paper coated with gelatin and treated with an ammoniacal solution of shellac in borax, on which wax or resin was rubbed. The advantage of this flexible support is that the dark parts of the picture have no tendency to contract from the lighter parts, which they were apt to do when a metal plate was used, as was the case in Johnson's original process. With this patent, and minor improvements made since, carbon-printing has arrived at its present state of perfection.
According to P. E. Liesegang, the carbon-tissue when prepared on a large scale consists of from 120 to 150 grains of gelatin (a soft kind), 15 grains of soap, 21 grains of sugar and from 4 to 8 rains of dry colouring matter. The last-named may be of various inds, from lamp-black igment to soluble colours such as alizarin. The gelatin, sugar an<fwap are put in water and allowed to stand for an hour, and then melted, the liquid afterwards receiving the
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 tne 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 aper being brought into contact with the surface of the elatin gy being made to revolve on the rollers. The thickness of t e coating is proportional to the rate at which the per is drawn over the gelatin: the slower the movement, the thic er the coatin . The paper is taken 05 the rollers, cut through, and hung up to ry 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 eflectcd by turning up about I in. from 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 d 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 iece of ordina silvered paper in an actinometer, or hotometer, a ongside the car n-tissue to ascertain the amount ofp light that has acted on it. There are several devices for ascertaining this amount, the simplest being an arran ement of a varying number of thicknesses of gold-beater's skin. he value of I, 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 ed es of the tissue begin to curl up, its surface and that of the flexibli together and placed flat. The water is pressed out with an in 1aru ber 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 sup ortAn application of warm water removes the rest of the so uble 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-pa r 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 a rain as a temporary support for other pictures. If a reversed negative 1e used the image may be transferred at once to its final support instead of to the temporary flexible su port, which is a point of practical value, since single-transfer are etter than double-transfer prints.
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 Prinlin Processes.—Poitevin claimed to have discovered that a film 0 r elatin impre nated with potassium bichromatc, after being acte upon by lig t and damping, would receive greasy ink.on those parts which had been aflected by light. But Paul Oreloth seems to have made the discovery rcvious to 1854, for in his patent of that year he states that his esigns were inked with printing ink before being transferred to stone or zinc. C. M. Tessie de Motay (in i 865) 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 procedureconsisted in coating metallic plates with gelatin impregnated with potassium our ammonium bichroniate or tri-chromate and mercuric chloride, then treatin with silver oleate, ex osing to light through a negative, washing, in ing 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
e support are brou ht'
the whole became coagulated rendered these unma eable. lt at_l_ast occurred to himthat if the hardening action 0 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 con uently 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 strip d 05 the plate and exposed in that manner. Albert adopted tii: 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 0 Munich, and had endeavoured to introduce a rocess which resembled Albert's earlier efforts. The name of “ Lic tdruck " 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 \V. \Voodbury 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. Mare Antoine A. Gaudin claimed the principle of the process, insisting that it was old, and basing his pretensions on the fact that he had rinted with translucent ink from intaglio blocks engraved by hand); but at the same time he remarked that the application of the princi le might lead to important results. It was just these results whic Woodbury obtained, and for which he was entitled to the fullest credit. oodbury subsequently introduced certain modifications, the outcome being what is known as the “ stannotype rocess," of which in 1880 he read a description before the French P otographic 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 a so inca ble of absorbing water where the action of the light has had ful play. It is this last phenomenon which occupies such an important place in photoithography. In the spring of [85 E. J Asser of Amsterdam produced photographs on a paper asis in printer's ink. Being anxious to roduce copies of such prints mechanically, be conceived the idea of) transferring the greasy ink impression to stone, and multiplyin the impressions by mechanical lithography. Following very closey upon Asser, J. W. Osborne of Melbourne made a similar application; his process is described by himself in the Phatogra hic Journal for April I860 as follows: “ A negative is produce 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 wa . Early in 1860 Colonel Sir H. ames, R.E-. F.R.S., brought ibrward the Southampton method 0 photo-lithe graphy, which had been carefully worked out by Captain de Come)~ Scott, R.E. The “ papyrotype rocess " was published by Abney in 1870 (see LITHOGRAPHY and ROCESS).
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 Farbmlehre 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. HCI’SChcl (Athenaeum, No. 62:) 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 drawin§ 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. Becquerei 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 to the current the plate took successively a grey, a. yellow, 3 violet and a blue tint, which order was again repeated. \Vhen tits violet tint appeared for the second time the plate was withdrawn and washed and dried over a spirit-lamp. In this state it