US3072480A

US3072480A - Photographic diffusion transfer process

Info

Publication number: US3072480A
Application number: US805451A
Authority: US
Inventors: Thomas I Abbott; Edward C Yackel
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1959-04-10
Filing date: 1959-04-10
Publication date: 1963-01-08
Anticipated expiration: 1980-01-08

Description

Jan. 8, 1963 1'. l. ABBOTT ETAL 3,072,480

PHOTOGRAPI-IIC DIFFUSION TRANSFER PROCESS Filed April 10. 1959 st g EXPOSURE DEVELOP/"5N7; TRANSFER OF HAL IDE ION HEAVY METAL SALT RECEPTION SHEET SILVER SAL T AND DEVELOPMENT THOMAS ABBOTT EDWARD C. VAC/(EL @2296 MC M ATTORNEY 8 AGENT States This invention relates to a process in photography wherein a product of development of photosensitive silver halide, the halide ion, is utilized to produce an image by transfer to a reception layer and conversion to a visible form. v

According to the invention a silver halide emulsion layer is exposed to a subject, then moistened with an alkaline solution in the presence of a silver halide developing agent present either in the alkaline solution or in the sensitive element itself, and when the emulsion layer is allowed to develop and is placed in contact with an absorbent reception surface such as paper, the halide ion produced in development migrates imagewise to the reception surface and is converted to a visible image thereon.

In one embodiment of the invention the silver halide developing agent used for developing the emulsion layer is initially present in the emulsion layer and development is initiated by application of an alkaline solution, the emulsion layer being placed into contact with an absorbent sheet to transfer the halide ion image thereto, following which the halide image is converted to a visible form, for example, by reaction with a heavy metalsalt followed by reaction with a soluble silver salt and the development of the resulting silver halide image. Alternatively, the transferred halide image may be reacted directly with a soluble silver salt and the resulting silver halide image developed to a silver image.

In a further embodiment of the invention the silver halide emulsion and reception layer are a part of a single photographic element, that is, the layers are coated one over the other. In this case the developing agent may or may not be present in the sensitive element and an alkaline solution is used to develop the emulsion and cause the transfer of the halide ion to the reception layer for conversion to a visible image.

One embodiment of our invention is illustrated in Stages 1 to 3 of the accompanying drawings wherein is shown in greatly enlarged cross-sectional view the appearance of a silver halide emulsion layer during exposure, development, and transfer of halide ion to a reception sheet and con version of the resulting heavy metal halide image to a visible silver image. Stage 1 shows the exposure of the sensitive element, Stage 2 the emulsion and reception layer in contact during the transfer of the halide ion image and Stage 3 the final silver print obtained from the halide ion image The following examples will serve to illustrate our invention:

Example 1 A photographic paper comprising a paper support and a gelatino silver bromoiodide emulsion layer containing the substantially nondiffusing silver halide developing agent 4-phenyl catechol, was exposed to a negative subject as shown in Stage 1 of the drawings wherein layer of the element represents the paper support and layer 11 the emulsion containing the developing agent, and area 12 the regionof exposure. Development of the emulsion was commenced by bathing it with a 4-percent aqueous solution of sodium carbonate for 10 seconds, followed by squeegeeing off the excess solution and pressing the emulsion into contact with the reception paper 13 (which had been treated with a 15 percent solution of lead acetate and atent dried) as shown in Stage 2 of the drawings. The halide ion formed in the development of area 12 of the emulsion diffuses imagewise to the reception sheet, reacting with the lead salt to form a lead halide image in area 14. The two elements were then stripped apart and after briefly washing the sheet with water, it was treated with a 10 percent solution of silver nitrate which converted the lead halide image to silver halide. After washing the sheet for 5 minutes it was exposed to light and developed at 68 F. for 2 minutes in the following developer composition:

. Grams Monomethyl-p-aminophenol sulfate 3 Sodium sulfite, desiccated 45 Hydroquinone 12 Sodium carbonate, monohydrate 80 Potassium bromide 2 Water to make 1 liter.

As a result of development, a silver image was formed in area 15, as shown in Stage 3 of the drawing. The image had high density and good definition.

Example 2 A photographic emulsion was exposed and treated with carbonate solution for 10 seconds as described in Example 1. The emulsion was then pressed into contact with a sheet of common onion skin paper with the result that the halide released in the development was absorbed imagewise by the paper fibers. The two elements were then stripped apart and the paper carrying the halide image was soaked for 1 minute in 10 percent aqueous silver nitrate solution and rinsed in water. The sheet now carrying a silver halide image was bathed in an oxidizing bath of 2 grams of potassium dichromate and 2 cc. of concentrated sulfuric acid per liter of water in order to remove silver present in the non-image areas thus leaving only a silver halide image on the sheet. After washing the sheet it was exposed to light and developed as described in Example 1 to obtain a positive silver image from the original negative. The silver image had high density and sharpness.

In a procedure similar to that of Example 1, after transfer of the halide image to the reception sheet containing heavy metal salt such as lead acetate, the resulting heavy metal halide image may be treated with sodium sulfide solution to convert the metal halide image to a colored metal sulfide image.

In the processes described, the silver halide developing agent used in the initial development step may be contained in the alkaline solution used for development rather than in the emulsion layer itself. The more soluble developing agents such as hydroquinone, monomethyl-paminophenol sulfate, 3-pyrazolidones, etc., are suitable for this purpose.

The silver halide of the emulsion layer is not especially critical, useful results being obtained with, for example, silver bromide, silver bromoiodide or silver chloride emuls1ons.

In the initial development step it is advantageous to use a minimum amount of agitation to insure that the halide ion does not difiuse through the emulsion layer to any great extent.

In a manner similar to the above examples, the processes can be carried out with a multilayer element having a support carrying a precipitant for halide ion such as lead salt and thereover a removable silver halide emulsion layer such as a cellulose ether phthalate silver halide emulsion layer. On development of the emulsion layer, halide ion migrates imagewise from the developed regions to the underlying layer where it reacts with the precipitant for the halide ion. At this stage the emulsion layer may be rubbed off or otherwise removed and the metal halide image remaining on the support may be treated as described above to increase its optical density.

The photographic emulsions used in practicing our invention are of the developing-out type.

The emulsions can be chemically sensitized by any of the accepted procedures. The emulsions can be digested with naturally active gelatin, or sulfur compounds can be added such as those described in Sheppard U.S. Patent 1,574,944, issued March 2, 1926, Sheppard et al. U.S. Patent 1,623,499, issued April 5, 1927, and Sheppard et a1. U.S. Patent 2,410,689, issued November 5, 1946.

The emulsions can also be treated with salts of the noble metals such as ruthenium, rhodium, palladium, iridium, and platinum. Representative compounds are ammonium chloropalladate, potassium chloroplatinate, and sodium chloropalladite, which are used for sensitizing in amounts below that which produces any substantial fog inhibition, as described in Smith and Trivelli U.S. Patent 2,448,060, issued August 31, 1948, and as anti-foggants in higher amounts,- as described in Trivelli and Smith U.S. Patents 2,566,245, issued August 28, 1951, and 2,566,263, issued August 28, 1951.

The emulsions can also be chemically sensitized With gold salts as described in Waller et a1. U.S. Patent 2,399,- 083, issued April 23, 1946, or stabilized with gold salts as described in Damschroder U.S. Patent 2,597,856, issued May 27, 1952, and Yutzy and Leermakers U.S. Patent 2,597,915, issued May 27, 1952. Suitable compounds are potassium chloroaurite, potassium aurithiocyanate, potassium chloroaurate, auric trichloride and 2- aurosulfobenzothiazole methochloride.

The emulsions can also be chemically sensitized with reducing agents such as stannous salts (Carroll U.S. Patent 2,487,850, issued November 15, 1949), polyamines, such as diethyl triamine (Lowe and Jones U.S. Patent 2,518,- 698, issued August 15, 1950), polyamines, such as spermine (Lowe and Allen U.S. Patent 2,521,925, issued September 12, 1950), or bisUB-aminoethyl) sulfide and its water-soluble salts (Lowe and Jones U.S. Patent 2,521,- 926, issued September 12, 1950).

The emulsions can also be optically sensitized with cyanine and merocyanine dyes, such as those described in Brooker U.S. Patents 1,846,301, issued February 23, 1932; 1,846,302, issued February 23, 1932; and 1,942,854, issued January 9, 1934; White U.S. Patent 1,990,507, issued February 12, 1935; Brooker and White U.S. Patents 2,112,140, issued March 22, 1938; 2,165,338, issued July 11, 1939; 2,493,747, issued January 10, 1950; and 2,739,- 964, issued March 27, 1956; Brooker and Keyes U.S. Patent 2,493,748, issued January 10, 1950; Sprague U.S. Patents 2,503,776, issued April 11, 1950; and 2,519,001, issued August 15, 1950; Heseltine and Brooker U.S. Patent 2,666,761, issued January 19, 1954; Heseltine U.S. Patent 2,734,900, issued February 14, 1956; Van Lare U.S. Patent 2,739,149, issued March 20, 1956; and Kodak Limited British Patent 450,958, accepted July 15, 1936.

The emulsions can also be stabilized with the mercury compounds of Allen, Byers and Murray U.S. Patent 2,- 728,663, issued December 27, 1955; Carroll and Murray U.S. Patent 2,728,664, issued December 27, 1955; and Leubner and Murray U.S. Patent 2,728,665, issued December 27, 1955; the triazoles of Heimbach and Kelly U.S. Patent 2,444,608, issued July 6, 1948; the azaindenes of Heirnbach and Kelley U.S. Patents 2,444,605 and 2,444,606, issued July 6, 1948; Heimbach U.S. Patents 2,444,607, issued July 6, 1948, and 2,450,397, issued September 28, 1948; Heimbach and Clark U.S. Patent 2,444,609, issued July 6, 1948; Allen and Reynolds U.S. Patents 2,713,541, issued July 19, 1955, and 2,743,181, issued April 24, 1956; Carroll and Beach U.S. Patent 2,- 716,062, issued August 23, 1955; Allen and Beilfuss U.S. Patent 2,735,769, issued February 21, 1956; Reynolds and Sagal U.S. Patent 2,756,147, issued July 24, 1956; Allen and Sagura U.S. Patent 2,772,164, issued November 27,

1956, and those disclosed by Birr in Z. wiss. Phot. vol. 47, 1952, pages 2-28; the disulfides of Kodak Belgian Patent 569,317, issued July 31, 1958; the quaternary benzothiazolium compounds of Brooker and Staud U.S. Patent 2,131,038, issued September 27, 1938, or the polymethylene bis-benzothiazolium salts of Allen and Wilson U.S. Patent 2,694,716, issued November 16, 1954; or the zinc and cadmium salts of Jones U.S. Patent 2,839,405, issued June 17, 1958.

The emulsions may also contain speed-increasing compounds of the quaternary ammonium type of Carroll U.S. Patent 2,271,623, issued February 3, 1942; Carroll and Allen US. Patent 2,288,226, issued June 30, 1942; and Carroll and Spence U.S. Patent 2,334,864, issued Novemoer 23, 1943; and the polyethylene glycol type of Carroll and Beach U.S. Patent 2,708,162, issued May 10, 1955.

The emulsions may contain a suitable gelatin plasticizer such as glycerin; a dihydroxy alkane such as 1,5- pentane diol as described in Milton and Murray U.S. application Serial No. 588,951, filed June 4, 1956; an ester of an ethylene bis-glycolic acid such as ethylene bis (methyl glycolate) as described in Milton U.S. application Serial No. 662,564, filed May 31, 1957; bis-(ethoxy diethylene glycol) succinate as described in Gray U.S. application Serial No. 604,333, filed August 16, 1956, or a polymeric hydrosol as results from the emulsion polymerization of a mixture of an amide of an acid of the acrylic acid series, an acrylic acid ester and a styrenetype compound as described in Tong U.S. Patent 2,852,- 386, issued September 16, 1958. The plasticizer may be added to the emulsion before or after the addition of a sensitizing dye, if used.

The emulsions may be hardened with any suitable hardener for gelatin such as formaldehyde; a halogen-substituted aliphatic acid such as mucobromic acid as described in White U.S. Patent 2,080,019, issued May 11, 1937; a compound having a plurality of acid anhydride groups such as 7,8-diphenylbicyclo (2,2,2)-7-octene-2,3,5,6-tetracarboxylic dianhydride, or a dicarboxylic or a disulfonic acid chloride such as terephthaloyl chloride or naphthalene-1,5-disulfonyl chloride as described in Allen and Carroll U.S. Patents 2,725,294, and 2,725,295, both issued November 29, 1955; a cyclic 1,2-diketone such as cyclopentane-1,2-dione as described in Allen and Byers U.S. Patent 2,725,305, issued November 29, 1955; a bisester of methane-sulfonic acid such as 1,2-di-(methane-sulfonoxy)-ethane as described in Allen and Laakso U.S. Patent 2,726,162, issued December 6, 1955; 1,3-dihydroxymethylbenzimidazol-2-one as described in July, (nott and Pollak U.S. Patent 2,732,316, issued January 24, 1956; a dialdehyde or a sodium bisulfite derivative thereof, the aldehyde groups of which are separated by 23 carbon atoms, such as B-methyl glutaraldehyde bis-sodium bisulfite as described in Allen and Burness U.S. Patent application Serial No. 556,031, filed December 29, 1955; a bis-aziridine carboxamide such as trimethylene bis(laziridine carboxamide) as described in Allen and Webster U.S. patent application Serial No. 599,891, filed July 25, 1956; or 2,3-dihydroxy dioxane as described in Jeffreys U.S. Patent 2,870,013, issued January 20, 1959.

The emulsions may contain a coating aid such as saponin; a lauryl or oleyl monoether of polyethylene glycol as described in Knox and Davis U.S. Patent 2,831,766, issued April 22, 1958; a salt of a sulfated and alkylated polyethylene glycol ether as described in Knox and Davis U.S. Patent 2,719,087, issued September 27, 1955; an acylated alkyl taurine such as the sodium salt of N-oleoyl-N-methyl taurine as described in Knox, Twardokus and Davis U.S. Patent 2,739,891, issued March 27, 1956; the reaction product of a dianhydride of tetracarboxybutane with an alcohol or an aliphatic amine containing from 8 to 18 carbon atoms which is treated With a base, for example, the sodium salt of the monoester of tetracarboxybutane as described in Knox,

Stenberg and Wilson U.S. Patent 2,843,487, issued July 15, 1958; a water-soluble maleopimarate or a mix ture of a water-soluble maleopimarate and a substituted glutamate salt as described in Knox and Fowler U.S. Patent 2,823,123, issued February 11, 1958; an alkali metal salt of a substituted amino acid such as disodium N-carbo-p-tert. octylphenoxypentaethoxy)glutamate as described in Knox and Wilson U.S. patent application Serial No. 600,679, filed July 30, 1956; or a sulfosuccinamate such as tetrasodium N-(1,2-dicarboxyethyl)-N- octadecyl sulfosuccinamate or N-lauryl disodium sulfosuccinamate as described in Knox and Stenberg U.S. patent application Serial No. 691,125, filed October 21, 1957.

In the preparation of the silver halide dispersions employed for preparing silver halide emulsions, there may be employed as the dispersing agent for the silver halide in its preparation, gelatin or some other colloidal material .such as colloidal albumin, a cellulose derivative, or a synthetic resin, for instance, a polyvinyl compound. Some colloids which may be used are polyvinyl alcohol or a hy drolyzed polyvinyl acetate as described in Lowe U.S. Patent 2,286,215, issued June 16, 1942; a far hydrolyzed cellulose ester such as cellulose acetate hydrolyzed to anacetyl content of 19-26% as described in U.S. Patent 2,327,808 of Lowe and Clark, issued August 24, 1943; a water-soluble ethanolamine cellulose acetate as described in Yutzy U.S. Patent 2,322,085, issued June 15, v1943; a polyacrylamine having a combined acrylamide content of 30-60% and a specific viscosity of 025+ 1.5 on an imidized polyacrylamide of like acrylamide content and viscosity as described in Lowe, Minsk and Kenyon U.S. Patent 2,541,474, issued February 13, 19,51; zein as described in Lowe U.S. Patent 2,563,791, issued August 7, 1951; a vinyl alcohol polymer containing urethane carboxylic acid groups of the type described in Unruh and Smith U.S. Patent 2,768,l54,-issued October 23, 1956; or containing cyano-acetyl groups such as the vinyl alcohol-vinyl cyanoacetate copolymer as described in Unruh, Smith and PriestUS. Patent 2,808,331, is sued October 1, 1957; or a polymeric material which results from polymerizing a protein or a saturated acylated protein with a monomer having a vinyl group as described in U.S. Patent 2,852,382, of Illingsworth, Dann and Gates, issued September 16, 1958.

If desired, compatible mixtures of two or more of these colloids may be employed for dispersing the silver halide in its preparation. Combinations of these antifoggants, sensitizers, hardners, etc., may be used.

What we claim is:

l. A photographic diffusion transfer process for produring negative images comprising I (a) exposing a developing-out type silver halide emulsion layer to a subject,

([2) imbibing in said exposed emulsion layer an aqueous alkaline solution while the silver halide of said emulsion layer is in contact with a silver halide developing agent, but in the substantial absence of silver halide solvent, thereby forming in said silver halide emulsion layer a visible image comprising negative silver and halide ion,

(c) diffusing said halide ion'from the negative image areas only of said emulsion layer to a contiguous, light-insensitive, absorbent layer containing a lead salt soluble in aqueous alkaline solutions leaving unexposed, undeveloped silver halide in said emulsion layer, thereby forming a lead halide latent image from said diffused halide ion, and

(d) converting the resultant lead halide latent image to a visible image.

2. A photographic diffusion transfer process for producing negative images comprising (a) exposin a dev oning-out type silver halide emulsion layer to a subject, (b) imbibing in said exposed emulsion layer an aqueous alkaline solution while the silver halide of said emulsion layer is in contact with a silver halide developing agent, but in the substantial absence of silver halide solvent, thereby forming in said silver halide emulsion layer a visible image comprising negative silver and halide ion,

(0) diffusing said halide ion from the negative image areas only of said emulsion layer to a contiguous, light-insensitive, absorbent layer containing a lead salt soluble in aqueous alkaline solutions leaving unexposed, undeveloped silver halide insaid emulsion layer, thereby forming a lead halide latent image from said diffused halide ion, and

(d) converting the resultant lead halide latent image to a visible image with a soluble silver salt.

3. A photographic diifusion transfer process for producing negative images comprising (a) exposing a developing-out type silver halide emulsion layer to a subject,

(b) imbibing in said exposed emulsion layer an aqueous alkaline solution while the silver halide of said emulsion layer is in contact with a silver halide developing agent, but in the substantial absence of silver halide solvent, thereby forming in said silver halide emulsion layer a visible image comprising negative silver and halide ion,

(c) diffusing said halide ion from the negative image areas only of said emulsion layer to a contiguous, light-insensitive, absorbent layer containing a lead salt soluble in aqueous alkaline solutions leaving unexposed, undeveloped silver halide in said emulsion layer, thereby forming a lead halide latent image from said diffused halide ion, and

(d) converting the resultant lead halide latent image to a visible image with a sulfide solution.

4. A photographic diffusion transfer process for producing negative images comprising (a) exposing a developing-out type silver halide emulsion layer to a subject,

(c) diffusing said halide ion from the negative image areas only of said emulsion layer to a contiguous, light-insensitive, absorbent layer leaving unexposed, undeveloped silver halide in said emulsion layer, and

(d) converting the resultant halide ion latent image to a visible image with soluble silver salt.

' 5. A photographic diffusion transfer process for producing negative images comprising ('a) exposing a developing-out type silver halide emulsion layer to a subject,

(0) diffusing said halide ion from the negative image areas only of said emulsion layer to a contiguous, light-insensitive, absorbent layer containing a lead salt soluble in aqueous alkaline solutions leaving unexposed, undeveloped silver halide in said emulsion layer, the lead halide being less soluble than said lead salt which is soluble in aqueous alkaline solutions, thereby forming a lead halide latent image from said difiused halide ion, and

(d) converting the resultant lead halide latent image to a visible image.

3,072,480 7 8 6. A photographic diffusion transfer process for prodiffusing said halide ion from the negative image ducing negative images comprising areas only of said emulsion layer to a contiguous,

(a) exposing a developing-out type silver halide emullight-insensitive, absorbent layer containing lead acesion layer to a subject, tate leaving unexposed, undeveloped silver halide in (b) imbibing in said'exposed emulsion layer an aqueous said emulsion layer, thereby forming a lead halide alkaline solution While the silver halide of said emullatent image from said diffused halide ion, and

sion layer is in contact with a silver halide developing (d) converting the resultant lead halide latent image agent, but in the substantial absence of silver halide to a visible image.

solvent, thereby forming in said silver halide emul- 9. A photographic diffusion transfer process for prosion layer a visible image comprising negative silver 10 ducing negative images comprising and halide ion, (a) exposing a developing-out type silver halide emul- (c) diffusing said halide ion from the negative image sion layer to a subject,

areas only of said emulsion layer to a contiguous, (b) imbibing in said exposed emulsion layer an aqueous light-insensitive, absorbent layer containing a lead salt soluble in aqueous alkaline solutions leaving unexposed, undeveloped silver halide in said emulsion layer, the lead halide being less soluble than said lead salt Which is soluble in aqueous alkaline solutions, thereby forming a lead halide latent image from said (c) diffusing said halide ion from the negative image areas only of said emulsion layer to a contiguous, light-insensitive, absorbent layer containing lead acetate leaving unexposed, undeveloped silver halide in said emulsion layer, thereby forming a lead halide latent image from said diffused halide ion, and

10. A photographic diffusion transfer process for producing negative images comprising (a) exposing a developing-out type silver halide emulsion layer to a subject,

(b) imbibing in said exposed emulsion layer an aqueous alkaline solution While the silver halide of said emulsion layer is in contact with a silver halide developing agent, but in the substantial absence of silver halide difiused halide ion, and

7. A photographic diffusion transfer process for producing negative images comprising (a) exposing a developing-out type silver halide emulsion layer to a subject,

(b) imbibing in said exposed emulsion layer an aqueous alkaline solution while the silver halide of said emulsion layer is in contact with a silver halide developing agent, but in the substantial absence of silver 0 halide solvent, thereby forming in said silver halide emulsion layer a visible image comprising negative silver and halide ion,

(c) diffusing said halide ion from the negative image areas only of said emulsion layer to a contiguous,

solvent, thereby forming in said silver halide emulsion layer a visible image comprising negative silver and halide ion,

(c) dilfusing said halide ion from the negative image areas only of said emulsion layer to a contiguous, light-insensitive, absorbent layer containing lead acetate leaving unexposed, undeveloped silver halide in said emulsion layer, thereby forming a lead halide latent image from said diffused halide ion, and

(d) converting the resultant lead halide latent image 8. A photographic diffusion transfer process for proto a visible image With sulfide solution.

ducing negative images comprising (a) exposing a developing-out type silver halide emulsion layer to a subject, (b) imbibing in said exposed emulsion layer an aqueous alkaline solution While the silver halide of said emul- References Cited in the file of this patent UNITED STATES PATENTS sion layer is in contact with a silver halide develop- 2698238 Land 1954 2,705,676 Land et al. Apr. 5, 1955 mg agent, but in the substantial absence of silver 2,712,995 Weyde July 12, 1955 halide solvent, thereby forming 1n said silver halide W M emulsion layer a visible ima comprisin" negative 2937945 eyde et ay 1960 Silver and halide 2,952,537 Blake et al Sept. 13, 1960

Claims

4. A PHOTOGRAPHIC DIFFUSION TRANSFER PROCESS FOR PRODUCING NEGATIVE IMAGES COMPRISING (A) EXPOSING A DEVELOPING-OUT TYPE SILVER HALIDE EMULSION LAYER TO A SUBJECT, (B) IMBIBING IN SAID EXPOSED EMULSION LAYER AN AQUEOUS ALKALINE SOLUTION WHILE THE SILVER HALIDE OF SAID EMULSION LAYER IS IN CONTACT WITH A SILVER HALIDE DEVELOPING AGENT, BUT IN THE SUBSTANTIAL ABSENCE OF SILVER HALIDE SOLVENT, THEREBY FORMING IN SAID SILVER HALIDE EMULSION LAYER A VISIBLE IMAGE COMPRISING NEGATIVE SILVER AND HALIDE ION, (C) DIFFUSING SAID HALIDE ION FROM THE NEGATIVE IMAGE AREAS ONLY OF SAID EMULSION LAYER TO A CONTIGUOUS, LIGHT-INSENSITIVE, ABSORBENT LAYER LEAVING UNEXPOSED, UNDEVELOPED SILVER HALIDE IN SAID EMULSION LAYER, AND (D) CONVENTING THE RESULTANT HALIDE ION LATENT IMAGE TO A VISIBLE IMAGE WITH SOLUBLE SILVER SALT.