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caused by the different thicknesses of the minutely thin film of iodide. The stage of maximum sensitiveness is obtained when it is of a golden orange colour. In this state the plate is withdrawn and removed to thedark slide of the camera, read for exposure. A plan frequently ado ted to give an even film 0 iodide was to saturate a card With iodine and hold the plate a short distance above the card. Long exposures were required, varying in Paris from three to thirty minutes. The length of the exposure was evidently a matter of judgment, more particularly as over-exposure introduced an evil which was called “ solarization," but which was in reality due to the oxidation of the iodide by prolonged ex sure to light.

As a matter of history it may be remarked that t e development of the ima e by mercury vapour is said to be due to a chance discovery of aguerre. It appears that for some time previous to the publication 0 the daguerreoty method he had been experimenting with iodized silver plates, p ucing ima es by what would now be called the “printing out ' process. T is operation involved so long an exposure that he sought some means of reducing it by the application ‘of different reagents. Having on one occasion exposed such a plate to a camera-image, he accidentally placed it in the dark in a cu board containing various chemicals, and found after the lapse o a night that he had a perfect im e developed. By the process of exhaustion he arrived at the fact t at it was the mercu vapour, which even at ordinary tem ratures volatilizes, that ha caused this intensification of the a most invisible camera-image. It was this discovery that enabled the exposu to be very considerably shortened from those which it was foun necessary to give in mere camera-Printing. .

The development of the image was effected by placing the exposed plate over a slightly heated (about 75° C.) cup of mercury. The vapour of mercury condensed on those places where the light had acted in an almost exact ratio to the intensity of its action. This produced a picture in an amalgam, the vapour of which attached itself to the altered silver iodide. Proof that such was the case was subsequently afforded by the fact that the mercurial image could be removed by heat. The developing box was so constructed that it was possible to'examine the icture through a yellow glass window whilst the image was being rought out. The next operation was to fix the picture by di pin it in a solution of hyposulphite of soda. The image roduced by t is method is so delicate that it will not bear the slig test handling, and has to be protected from accidental touching.

The first great improvement in the daguerreotype process was the resensitizing of the iodized film by bromine vapour. John Frederick Goddard published his account of the use of bromine in conjunction with iodine in 1840, and A. F. I. Claudet (1797—1867) employed a combination of iodine and chlorine vapour in 1841. In 1844 Daguerre published his improved method of preparing the plates, which is in reality based on the use of- bromine with iodine. That this addition points to additional sensitiveness will be readily understood when we remark that so-ca'lled instantaneous pictures of yachts in full sail, and of large size, have been taken on plates so prepared—a feat which is utterly impossible with the original process as described by Daguerre. The next improvement in the process was toning or gilding the image by a solution of gold, a practice introduced by H. L. Fizeau. Gold chloride is mixed with byposulphite of soda, and the levelled plate, bearing a. suflicient quantity of the fluid, is warmed by a spirit-lamp until the required vigour is given to the image, as a. consequence of which it is better. seen in most lights. Nearly all the daguerreotypes extant have been treated in this manner, and no doubt their permanence is in a great measure due to this operation. Images of this class can be copied by taking electrotypes from them, as shown by Sir W. R. Grove and others. - These reproductions are admirable in every way, and furnish a proof that the daguerrean image is a relief.

Fox-Talbot Process—In January 1839 Fox Talbot described the first of his processes, photogenic drawing, in a paper to the Royal Society. He states that he began experimenting in 1834, and that in the solar microscope he obtained an outline of the object to be depicted in full sunshine in half a second. He published in the Philosophical Magazine full details of his method, which consisted essentially in soaking paper in common salt, brushing one side only of it with about a 12% solution of silver nitrate in water, and drying at the 'fire. Fox Talbot stated that by repeating the alternate washes of the, silver and salt—~always ending, however, with the former—greater sensitiveness was attained. This is the same in every respect as the method practised byr‘Wedgwood in 1802; but, when we come

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to the next process, which he called “ calotype ” or “ beautiful picture,” we have a distinct advance. This process Talbot protected by a patent in i84i.

It may be briefl described as the application of silver iodide to a paper sup rt. arefully selected paper was brushed over with a solution 0 silver nitrate (100 grains to the ounce of distilled water), and dried by the fire. It was then dipped into a solution of potassium iodide (500 grains being dissolved in a pint of water), where it was allowed to stay two or three minutes until silver iodide was formed. In this state the iodide is scarcely sensitive to light, but is sensitized by brushing “ gallo-nitrate of silver " over the surface to which the silver nitrate had been first applied. This “ allonitrate " is merely a mixture, consisting of 100 grains of silver nitrate dissolved in 2 oz. of water, to which is added one-sixth of its volume of acetic acid, and immediately before applying to the pager an equal bulk of a saturated solution of gallic aCid in water. T e prepared surface is then ready for exposure in the camera, and, after a short insolation, develops itself in the dark, or the development may be hastened b a fresh application of the “ gallo-nitrate of silver." The

icture is then fixed by washing it in clean water and drying slightly in blotting aper, after which it is treated with a solution of potassium bromi e, and again washed and dried. Here there is no mention made of hyposul hite of soda as a fixing agent, that having been first used by Sir . Herschel in February 1840.

In a strictly historical notice it ought to be mentioned that development by means of gallic acid and silver nitrate was first known to Rev. J. B. Reade. W'hen impressing images in the solar microscope he employed gallic acid and silver in order to render more sensitive the silver chloride paper that he was using, and he accidentally found that the image could be developed without the aid of light. The priority of tie discovery was claimed by Fox Talbot; and his claim was sustained after a lawsuit, a parently on the ground that Reade's method had never been legal y published. Talbot afterwards made many slight improvements in the rocess. In one of his tents he recognizes the value of the proper xing of his photogenic drawings by h posulphite of soda, and also the production of positive prints rom the calotypc negatives. We pass over his application of albumen to porcelain and its subsequent treatment with iodine vapour, as also his application of albumen in which silver iodide was held in suspension to a glass plate, since in this he was preceded by N iepce de St Victor in 1848.

Albumen Process on Glass.—It was a decided advance when Niepce de St Victor, a nephew of Nicéphore de Niepce, employed a glass plate and coated it with iodized albumen. The originator of this method did not meet with much success. In the hands of Blanquart Evrard it became more practicable; but it was carried out in its greatest perfection by G. Le Gray.

The outline of the operations is as follows: The whites of five fresh eggs are mixed With about one hundred grains of potassium iodide, about twenty grains of potassium bromide and ten rains of common salt. The mixture is beaten up into a froth and al owed to settle for twenty-four hours, when the clear liquid is decanted off. A circular pool of albumen is poured on a glass plate, and a straight ruler (its ends being wrapped with waxed paper to prevent its ge from touching the plate anywhere except at the margins) is drawn ‘over the plate, sweepin off the excess of albumen, and so leaving an even film. The p ate is first allowed to dry spontaneousl , a final heating being given to it in an oven or before the fire. The eat hardens the albumen, and it becomes insoluble and ready for the silver nitrate bath. One of the difficulties is to prevent gystallization .of the salts held in solution, and this can only be ected by keeping them in defect rather than in excess. The plate is sensitized for five minutes in a bath of silver nitrate, acidified with acetic acid, and exposed whilst still wet, or it may be sli htly washed and again dried and exposed whilst in its desiccat state. The image is developed by gallic acid in the usual way.

After the application of albumen many modifications were introduced in the shape of starch, serum of milk, gelatin, all of which were intended to hold iodide in silu on the plate; and the development in every case seems to have been by gallic acid. At one time the waxed-paper process subsequently introduced by Le Gray was a. great favourite. Paper that had been made translucent by white wax was immersed in a. solution of potassium iodide until impregnated with it, after which it was sensitized in the usual way, development being by gallic acid. In images obtained by this process the high lights are represented by metallic silver, whilst the shadows are translucent. Such a print is called a. “negative.” When silver chloride paper is darkened by the passage of light through a negative, we get the highest lights represented by white paper and the shadows by darkened chloride. A print of this kind is called a “ positive.”

C alludion Process—A great impetus was given to photography in 1850, on the introduction of collodion (q.v.), a very convenient vehicle on account of the facility with which the plates are prepared, and also because it is a substance as a rule totally unaffected by silver nitrate, which is not the case with other organic substances. Thus albumen forms a definite silver compound, as do gelatin, starch and gum. The employment of collodion was first suggested by Le Gray, but it remained for Frederick Scott Archer of London, closely followed by P. W. Fry, to make a really practical use of the discovery. When collodion is poured on a glass plate it leaves on drying a hard transparent film which under the microscope is slightly reticulated. Before drying, the film is gelatinous and perfectly adapted for holding in situ salts soluble in ether and alcohol. Where such salts are present they crystallize out when the film is dried, hence such a film is only suitable where the plates are ready to be immersed in the silver bath. As a rule, about five grains of the soluble gun-cotton are dissolved in an ounce of a mixture of equal parts of ether and alcohol, both of which must be of low specific gravity, ~72 5 and -805 respectively. If the alcohol or ether be much diluted with water the gun-cotton (pyroxylin) precipitates, but, even if less diluted, it forms a film which is “ crapey ” and uneven. Such was the material which Le Gray proposed and which Archer brought into practical use. The opaque silver plate with its one impression was abandoned; and the paper support of Talbot, with its inequalities of grain and thickness, followed suit, though not immediately. When once a negative had been obtained with collodion on a glass plate— the image showing high lights by almost complete opacity and the shadows by transparency (as was the case, too, in the calotype process)—any number of impressions could be obtained by means of the silver-printing process introduced by Fox Talbot, and they were found to possess a delicacy and refinement of detail that certainly eclipsed the finest print obtained from a calotype negative. To any one who had practised the somewhat tedious calotype process, or the waxed-paper process of Le Gray with its still longer preparation and development, the advent of the collodion method must have been extremely welcome, since it effected a saving in time, money and uncertainty. The rapidity of photographic action was much increased, and the production of a diflerent character of pictures thus became possible.

We 've an outline of the procedure. A glass plate is carefully cleane by a detergent such as a cream of tri li powder and spirits of wine (to which a little ammonia is often ad ed), then wiped with a soft rag, and finally polished with a silk handkerchief or chamois leather. A collodion containing soluble iodides and bromides is made to flow over the plate, all excess being drained off when it is covered. A good standard formula for the collodion is—55 grains of pyroxylin, 5 oz. of alcohol, 5 oz. of ether; and in this liquid are dissolved 2} , grains of ammonium iodide, 2 grains of cadmium iodide and 2 grains of cadmium bromide. When the collodion is set the plate is immersed in a bath of silver nitrate—a vertical form being that mostly used in England, whilst a horizontal dish is used on the continent of Europe—a good formula for which is 50 grains of silver nitrate with 10 oz. of water. The plate is steadi y lowered into this solution, and moved in it until all the repellent action between the aqueous solution of the silver and the solvents of the collodion is removed, when it is allowed to rest for a couple of minutes, after which period it is taken out and placed in the dark slide ready for exposure in the camera. After undergoing proper exposure the plate is withdrawn, and in a room lighted with yellow light the developing solution is applied, which originally was a solution of pyrogallic acid in water restrained in its action b the addition of acetic acid. One of the old formulae employed b . H. Delamotte was 9 grains of pyrogallic acid, 2 drachms of gacial acetic acid and 3 oz. of water. The image gradually ap rs after the application of this solution, building itself up from t esilver nitrate clinging to the film, which is reduced to the metallic state by degrees. Should the density be insufficient a few drops of silver nitrate are added to the pryogallic acid solution and the developing action continued. _

In 1844 Robert Hunt introduced another reducing agent, which is still the favourite, viz. ferrous sulphate. 13 its use the time of necessary exposure of the plate is reduced an the image develops with great rapidity. A sample of this developing solution is 20 grains of ferrous sul hate, 20 minims of acetic acid, with 1 oz. of water. This often eaves the image thinner than is requisite for the formation of a ood print, and it is intensified with pyrogallic acid and silver. Ot er intensifiers are used to increase the deposit on a plate by means of mercury or uranium, followed by other solutions to still further darken the double salts formed on the film.

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Such intensifyin a ents have to be applied to the image after the plate is fixed, w ic is done by a concentrated solution of hyposulphite of soda or b potassium cyanide, the latter salt having been first introduced by artin and Marc Antoine Augustin Gaudin in 1853 (La Lumiere, April 23, I853). Twenty-five rains of potassium cyanide to one ounce of water is the strength of tie solution usuall employed. The reaction of both these fixm agents is to form with the sensitive salts of silver double h sulp ites or cyanides, which are soluble in water and salt. The utility of bromides in the collodion process seems to have been recognized in its earliest days, Scott Archer (1852) and R. J. Bingham (i850) both mentioning it. We notice this, since as late as i866 a patent-right in its use was sought to be enforced in America, the patent being taken out by James Cutting in July 1854.

Positive Pictures by the Callodion Process.—In the infant! of the collodion process it was shown by Home that a negative image could be made to assume the appearance of a positive by whitening the metallic silver deposit. This he efiected by using with the pyrogallic acid developer a small quantity of nitric acid. A better result was obtained by P. W. Fry with ferrous sulphate and ferrous nitrate, whilst Hugh Diamond gave effect to the matter in a practical way. I F. Scott Archer used mercuric chloride to whiten the image. To Robert Hunt, however, must be rewarded the credit of noticing the action of this salt on the image (Phil. T vans, 1843). Thewhitened picture may be made to stand out against black velvet, or black varnish may be poured over the film to give the necessary black background, or, more recently, the positive pictures may be produced on japanned iron plates (ferrotype plates) or on japanned leather. This process is still occasionally practised by itinerant photographers.

Moist Collodion Process—It is seen that for the successful working of the collodion process it was necessary that the plate should be exposed Very shortly after its preparation; this was a drawback, inasmuch as it necessitated taking a heavy equipment into the field. In 1856, Sir William Crookes and J. Spiller published in the Philosophical Magazine a process whereby they were enabled to keep a film moist (so as to prevent crystallization of the silver nitrate) several days, enabling plates to be prepared at home, exposed in the field, and then developed in the dark room. The plate was prepared in the usual way and a solution of zinc nitrate and silver nitrate in water was made to flow over it. The hygroscopic nature of the zinc salt kept sufficient moisture on the plate to attain the desired end. Various modifications in procedure have been made.

Dry Plates—It would appear that the first experiments with collodion dry plates were due to Marc Antoine Augustin Gaudin. In La Lumiére of the 22nd of April and the 27th of May 1854 he describes his researches on the question; whilst in England G. R. Muirhead, on the 4th of August r854, stated that light acts almost as energetically on a dry surface as on a wet after all the silver has been washed away from the former previous to desiccation. J. M. Taupenot, however, Seems to have been the first to use a dry-plate process that was really workable. His original plan was to coat a plate with collodion, sensitize it in the ordinary manner, wash it, cause a solution of albumen to flow over the surface, dry it, dip it in a bath of silver nitrate acidified with acetic acid, and wash and dry it again. The plate was then in a condition to be exposed, and was to be developed with pyrogallic acid and silver. In this method we have a double manipulation, which is long in execution, though perfectly eflective.

A great advance was made in all dry-plate processes by the introduction of what is known as the “alkaline developer," which is, however, inapplicable to all plates on which silver nitrate is present in the free state. The developers previously described, either for collodion or paper processes, were dependent on the reduction of metallic silver by some such agent as ferrous sulphate, the reduction taking place gradually and the reduced particles aggregating on those portions of the film which had been acted upon by light. The action of light being to reduce the silver iodide, bromide or chloride, these reduced particles really acted as nuclei for the crystallized metal. It will be evident that in such a method of development the molecular attraction acts at distances relatively great compared with the diameters of the molecules themselves. If it were possible to reduce the altered particles of silver salt it was plain that development would be more rapid, and also that the number of molecules reduced by light would be smaller if the metallic silver could be derived from silver compounds within shorter distances of the centres of molecular attraction. Alkaline development accomplished this to a very remarkable extent; but the method is only really practicable when applied to films containing silver bromide and chloride, as silver iodide is only slightly amenable to the alkaline development. The introduction of this developer is believed to be of American origin; and it is known that in the year 1862 Major C. Russell used it with the dry plates he introduced.

An alkaline developer consists of an alkali, a reducing agent and a restraining a ent. These bodies, when combined and applied to the solid silver romide or chloride, after being acted upon by light, were able to reduce the sub-bromide or sub-chloride, and to build up an image upon it, leaving the unaltered bromide intact, except so far as it was used in the building up. In 1877 Sir W. Abney investigated this action. A dry plate was prepared by the bath process in the usual manner (to be described below), and exposed in the camera. The exposed film was covered with another film of collodiobromide emulsion, which of course had not seen the li ht. An image was obtained from the double film by means 0? the alkaline develo r, which penetrated through the upper unexposed film. The evelopment was prolonged until an image appeared through the unexposed film, when the plate was fixed, washed and dried. A piece of gelatinous pa or was cemented on the upper film, and a similar iece on the ower after both had been stripped off the glass. W en quite d the two apers were forcibly separated, a film adhering to each. he upper fi m, although never exPosed to light, showed an image in some cases more intense than the under film. The action of the alkaline developer was here manifest: the silver bromide in close contiguit t0 the exposed particles was reduced to the metallic state. lzlence, from this and similar experiments, Abney concluded that silver bromide could not exist in the presence of a freshly precipitated or reduced metallic silver, and that a sub-bromide was immediately formed. From this it will be seen that the de sited silver is well within the sphere of molecular attraction, an that consequentlyaless exposure (Le. the reduction of fewer molecules of the sensitive salt) would give a developable image.

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Solution B. Sodium carbonate . . 100 parts. Sodium sul hite . 125 ,, Potassium romide . . . 3 ,, Water 1000 ,,

A and B solutions are mixed together in equal proportions. Besides these, there are several more, such as adurol, glycin, rocatechin, which have been used with more or less success. hey all give a black in lieu of that dark olive-green deposit of

silver which is so often found with pyr allol developers. All are alkaline developers, and the image is bui t up from'the sensitive salt within the film. They are applicable to elatin or collodion plates, but for the latter rather more bromide 0 an alkali is added, to retard fogging.

Another set of developers for dry plates dependent on the reduction of the silver bromide and the metallic state is founded on the fact that certain organic salts of iron can be utilized. In r877 M. Carey Lea of Philadelphia and William Willis announced almost simultaneously that a solution of ferrous oxalate in neutral potassium oxalate was effective as a developer, and from that time its use has been acknowledged. In 1882 J. M. Eder demonstrated that gelatino-silver chloride plates could be developed with ferrous citrate, which could not be so readily accomplished with ferrous oxalate. The exposure for chloride plates when developed by the latter was extremely prolonged. In the same year Abney showed that if ferrous oxalate were dissolved in potassium citrate a much more powerful agent was formed, which allowed not only gelatino-chloride plates to be readily developed but also collodio-chlon'de plates. These plates were undevelopable except by the precipitation method until the advent of the agents last-mentioned owing to the fact that the chloride was as readily reduced as the sub-chloride.

Amongst the components of an alkaline developer we mentioned a restrainer. This factor, generally a bromide or chloride of an alkali, serves probably to form a compound with the silver salt which has not been acted upon by light, and which is less easily reduced than is the silver salt alone—the altered particles being left intact. The action of the restrainer is regarded by some as due to its combination with the alkali. But whichever theory is correct the fact remains that the restrainer does make the primitive salt less amenable to reduction. Such restrainers as the bromides of the alkalis act through chemical means; but there are others which act through physical means, an example of which we have in the preparation of a gelatin plate. In this case the gelatin wraps up the particles of the silver compound in a colloidal sheath, as it were, and the developing solution only gets at them in a very gradual manner, for the natural tendency of all such reducing agents is to attack the particles on which least work has to be expended. In the case of silver sub-bromide the developer has only to remove one atom of bromine, whereas it has to remove two in the case of silver bromide. The sub-bromide formed by light and that subsequently produced in the act of development are therefore reduced. A large proportion of gelatin compared with the silver salt in a film enables an alkaline developer to be used without any chemical restrainer; but when the gelatin bears a small proportion to the silver such a restrainer has to be used. With collodion films the particles of bromide are more or less unenveloped, and hence in this case some kind of chemical restrainer is absolutely necessary. We may say that the organic iron developers require less restraining in their action than do the alkaline developers.

In Major Russell's process the plate was prepared by immersion in a strong solution of silver nitrate and then washed and a preservative applied. The last-named agent executes _two functions, one beinglto a sorb the halogen liberated by the action of light and the ot er to preserve the film from atmos heric action. Tannin, which Major Russell employed, if we mista e not, is a good absorbent of the halogens, and acts as a varnish to the film. Other collodion dry-plate processes carried out by means of the silver-nitrate bath were very numerous at one time, many different organic bodies being also employed. In most cases ordina iodized collodion was made use of, a small percentage of solublezromide being as a rule added to it. When lates were developed by the alkaline method this extra bromid)e induced density, since it was the silver bromide alone which was amenable to it, the icdide being almost entirely unaffected t by the weak devaloper which was at that time in general use.

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Dry-Plate Bath Process.—One of the most successful bath dry-plate processes was introduced by R. Manners Gordon. The plate was given an edging of albumen and then coated with ordinary iodized collodion to which one grain per ounce of cadmium bromide had been added. It was kept in the silvernitrate bath for ten minutes, after which it was washed thoroughly. The following preservative was then applied:—

Gum arabic 20 grs. 1- Sugar candy 5 ,,

Water 6 dr. 2, 3 Gallic acid . . . . . 3 grs.

Water . . . . . . 2 dr.

These ingredients were mixed just before use and, after filtering, applied for-one minute to the plate, which was allowed to drain and set up to dry naturally. Great latitude is admissible in the exposure; it should rarely be less than four times or more than twenty times that which would be required for a wet plate under ordinary circumstances. The image may be developed with ferrous sulphate restrained by a solution of gelatin and glacial acetic acid, to which a solution of silver nitrate is added just before application, or by an alkaline developer.

In photographic processes not only has the chemical condition of the film to be taken into account but also the optical. When light falls on a semi-opaque or translucent film it is scattered by the particles in it and passes through the glass plate to the back. Here the rays are partly transmitted and partly reflected, a ve small quantity of them being absorbed by the material of the glass. heory points out that the strongest reflection from the back of the glass should take place at the “ critical " angle). In 18765 Abney investigated the subject and proved that practice a ree wrth theory in every respect, and that the image of a point of ight in development on a plate was surrounded by a ring of reduced silver caused by the reflection of the scattered light from the back surface of the glass, and that this ring was shaded inwards and outwards in such a manner that the shading varied with the intensit 0f the light reflected at different angles. To avoid “ halation,‘ as this phenomenon is called, it was usual to cover the back of dry plates with some material which should be in optical contact with it, and as nearly as possible of the same density asglass, and which at the same time should absorb all the photographically active rays. This was called “ backing a plate."

Collodion Emulsion Processes.—In 1864 W. B. Bolton and B. J. Sayce published the germ of a process which revolutionized photographic manipulations. In the ordinary collodion process a sensitive film is procured by coating a glass plate with collodion containing the iodide and bromide of some soluble salt, and then, when set, immersing it in a solution of silver nitrate in order to form silver iodide and bromide in the film. The question that presented itself to Bolton and Sayce was whether it might not be possible to get the sensitive salts of silver formed in the collodion whilst liquid, and a sensitive film. given to a plate by merely letting this collodion, containing the salts in suspension, flow over the glass plate. Gaudin had attempted to do this with silver chloride, and later G. W. Simpson had succeeded in perfecting a printing process with collodion containing silver chloride, citric acid and silver nitrate; but the chloride until recently has been considered a slow working salt, and nearly incapable of development. Up to the time of W. B. Bolton and B. J. Sayce’s experiments silver iodide had been considered the staple of a sensitive, film on which to take negatives; and though bromide had been used by Major Russell and others, it had not met with so much favour as to lead to the omission of the iodide. At the date mentioned the suspension of silver iodide in collodion was not thought practicable, and the inventors of the process turned their attention to silver bromide, which they found could be secured in such a fine state of division that it remained suspended for a considerable time in collodion, and even when precipitated could be resuspended 'by simple agitation. The outline of the method was to dissolve a soluble bromide in plain collodion, and add to it drop by drop an alcoholic solution of silver nitrate, the latter being in excess or defect according to the will of the operator. To prepare a sensitive surface the collodion containing the emulsified sensitive salt was poured over a glass plate, allOWed to set, and washed till all the soluble salts resulting from the double decomposition of the soluble bromide and the silver nitrate, together with the unaltered soluble bromide or silver

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nitrate, were removed, when the film was exposed wet, or allowed to dry and then exposed. The rapidity of these plates was not in any way remarkable, but the process had the great advantage of doing away with the sensitizing nitrate of silver bath, and thus avoiding a tiresome operation. The plates were developed by the alkaline method, and gave images which, if not primarily dense enough, could be intensified by the application of pyrogallic acid and silver nitrate as in the wet collodion process. Such was the crude germ of a method which was destined to effect a complete change in the aspect of photographic negative taking‘; but for some time it lay dormant. In fact there was at first much to discourage trial of it, since the plates often became veiled on development.

M. Carey Lea of Philadelphia, and W. Cooper, jun., of Reading, may be said to have given the real impetus to the method. Carey Lea, by introducing an acid into the emulsion, established a practicable collodion emulsion rocess, which was rapid and at the same time gave negative pictures ree from veil. To secure the rapidity Carey Lea employed a fair excess of silver nitrate, and Colonel H. Stuart Wortley gained further ra idity by a still greater increase of it; the free use of acid was t e only means by which this could be effected without hopelessly spoilin the emulsion. The addition of the mineral acids such as Carey ea employed is to prevent the formation of (or to destroy when formed) any silver sub-bromide or oxide, either of which acts as a nucleus on which development can take Iplace. Abney first showed the theoretical efi‘ect of acids on the su bromide, as also the effect of oxidizing agents on both the above com ounds (see below). A more valuable modification was introducedpin 1874 by W. 13. Bolton, one of the originators of the process, who allowed the ether and the alcohol of the collodion to evaporate, and then washed away all the soluble salts from the gelatinous mass formed of yroxylin and sensitive salt. After washing for a considerable time, the pellicle was dried naturally or washed with alcohol, and then the pyroxylin redissolved in ether and alcohol, leaving an emulsion of silver bromide, silver chloride or silver iodide, or mixtures of all suspended in collodion. In this state the plate could be coated and dried at once for ex sure. Sometimes, in fact generally, preservatives were used, as in t e case of dry plates with the bath, in order to prevent the atmosphere from rendering the surface of the film spotty or insensitive on development. This modification had the great advantage of allowing a large quantity of sensitive salt to be repared of precisely the same value as to rapidity of action and qua it of film.

A great advance in the use of the collbdion bromide process was made by Colonel Stuart Wortlcy, who in June 1873 made known the powerful nature of a strongly alkaline developer as opposed to the weak one which up to that time had usually been employed for a collodion emulsion late, or indeed for any ((131 plate.

An example of tlie preparation of a coll ion emulsion and the developer is the following: 2} oz. of alcohol, 5 oz. of ether, 75 ins of pyroxylin. In I 02. of alcohol are dissolved 200 grains 0 zinc bromide 1; it is then acidulated with 4 or 5 drops of nitric acid, and added to half the above collodion. In 2 drachms of water are dissolved 3 o grains of silver nitrate, I 02. of alcohol being added. The silver alcohol is next poured into the other half of the collodion and the brominized collodion drop in, care being taken to shake between the operations. An emu sion of silver bromide is formed in suspension; and it is in every case left for 10 to 20 hours to what is technically called “ ripen," or, in other words, to become creamy when (poured out upon a glass plate. When the emulsion has ripene it may be used at once or be poured out into a flat dish and the solvents allowed to evaporate till the p roxylin becomes gelatinous. In this state it is washed in water ti l all the soluble salts are carried away. After this it may be either spread out on a cloth and dried or treated with two or three doses of alcohol, and then redissolved in equal parts of alcohol (s ific gravity, ~805) and ether (specific gravity, -720). In this con ition it is a washed emulsion, and a glass plate can be coated with it and the film dried, or it may be washed and some of the many preservatives, such as albumen, beer, coffee, gum, &c., applied.

f lil‘he type of a useful alkaline developer for collodion plates is as o owe:—

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