US2458168A - Production of light-polarizing images - Google Patents

Description

Jan. 4, 1949. H. P. 'HUSEK v 2,458,168

Y PRODUCTION 4 OF, LIGHT-POLARIZING IMAGES I Filed Aug. 27. 1945 Iodine Image Moleculqry Oriefifed Currier Supporf.v

IN V EN TOR.

Mg/gm Patented Jan. 4, 1949 PRODUCTION "OF LIGHT-POLARIZING IIWAGES Helen P. Husek, Allston, Mass, assignor to.

Polaroid Corporation, Cambridge, Mass, a corporation of Delaware Application August 27, 1945, Serial No. 612,876 Claims. (01. 95- 88) This invention relates to photography and more particularly has reference to the processing of silver images to obtain iodine images of desired density and to products formed in such processing.

A method heretofore practiced for the formation of an iodine image converts asilver ferrocyanide and reacts the latter with a salt of a polyvalent metal in its higher valent form to provide a water-insoluble metallic .ferrocyanide which is converted to its corresponding waterinsoluble hydroxide upon reaction with a base. The Water-insoluble hydroxide thus produced is an oxidizing agent which when treated with an iodide in the presence of an acid will effect the release of iodine to provide an image in iodine, the metallic hydroxide .being dissolved during this reaction.

An iodine image formed in this manner is apt to have a greater density than the silver image from which it was derived. It is possible to avoid this undesirable over-densification of the iodine image by exposing the emulsion in which the silver image is formed to the end of providing a silver image of low density. For the amateur or novice, the practice involving the formation of an image of low density is in general difficult to carry out, especially when depending on visual controls.

Objects of the invention reside in the provision of processes for the formation from silver images of iodine images of desired density; for the control of the density of an iodine image in its formation from a silver image; and for the transformation of silver images in photographic carrier materials to iodine images of selected density.

Other objects of the invention relate to the provision of methods for treating silver images to form iodine images of desired density in photographic carriers of hydrophilic material such as gelatin and especially in transparent, linear polymeric plastics of the character which may have their molecules oriented, as for example polyvinyl alcohol; and also for the formation of dichroic iodine images of predetermined density in molecularly oriented materials whereby to provide veetograph images, that is to say images the contrast of which is a function of the direction of incident light; to employ processes of the character described wherein the photosensitive material is silver halide; and to provide processes for the production in a photographic carrier of an image containing an oxidizing agent which will controllably effect the release of iodine from an acid solution of an iodide.

Further objects of the invention relate to the provision in processes of the character described of thetreatmen-t of a silver image with a mixture of two or more metallic salts, each capable of forming water-insoluble ferrocyanides and each having comparable solubility products, at least one of the salts being asalt of a polyvalent metal:

he element in higher valent form and at least another of said salts being a salt of a univalent metallic element other than silver or a salt of a polyvalent metallic element in a lower valent form; and to provide processes for the DIOdLlCLlOIl of iodine images of controlled density from silver images by the use of at least one salt of a univalent metallic element other than silver orof a polyvalent metallic element in a lower valent form which is predeterminedly added to a treating solution for the silver image containing one or more salts of a polyvalent metallic element in higher valent form.

Still further objects of the invention reside in the product obtained by treatment of a silver image inan image-bearing layer with at least one salt of a polyvalent metallic element in higher valent form and at least one salt of a univalent metallic element other than silver or a salt of a polyvalent metallic element in lower valent form; and to a product comprising a transparent, molecularly orient-ed, high polymer having predetermined portions thereof containing a relatively water-insoluble compound of a polyvalent material in higher Valent form and also a relatively water-insoluble compound incapable of releasing iodine from an iodide solution the lastnamedcompound being a compound of .a polyvalent material in lower valent form or a compound 'of a univalent material.

The invention accordingly comprises the several steps and relation and order of one or more of such steps with respect to each of the others, and the article possessing the features, properties and the relation of elements, which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should 'be had to the following detailed description taken in connection with the accompanying drawing which is a schematic View of a photographic film having an iodine image formed therein and with which the invention may be practiced.

As indicated, the invention contemplates the formation of iodine images in image-bearing carrlers or layers of suitable material. By one proformed may comprise any photographic carrier material. Materials used for image-bearing layers may be transparent and may be described as hydrophilic in that they have a strong affinity for water or absorb or adsorb water. At the same time the carrier materials used with the invention may be termed relatively water-insoluble, reference being had to hydrophilic carriers having solubilities which will prevent water solutions used in processing from appreciably dissolving them during the time required to carry out the photographic treatments described herein. Preferred examples of carrier materials comprise gelatin and transparent, linear, polymeric plastics capable of having their molecules oriented as well as photographic printing papers.

Polymeric plastics, which are able to form a dichroic sorption complex with iodine and which possess a relatively high tensile strength so as to perm-itthe orientation of the molecules thereof,

are especially useful in carrying out certain embodiments of the invention. Materials falling within this description and possessing sufiicient strength to permit orientation of the molecules thereof are herein termed orientable plastics, all other materials being deemed to be nonorientable.

Molecularly oriented plastics when stained with a dichroic stain such as iodine are rendered light-polarizing. However, if orientable materials of this character are used in a substantially unoriented condition, iodine, while staining them will not render them light-polarizing. Thus, a

light-polarizing image may be formed by staining predetermined portions of a molecularly oriented plastic sheet with iodine. Also a lightpolarizing image may be obtained by the formation of an iodine image in an unorientable material such as gelatin which is mounted upon a base or support formed of a molecularly oriented plastic, the light-polarizing image being formed in the plastic by the migration of iodine from the gelatin.

Polymers characterized by having a multiplicity of hydroxy groups extending off the main polymer chain and consisting of simple repetitive grouping or units are generally useful as carriers for light-polarizing iodine images. Examples thereof are polyvinyl alcohol, polyhydroxy alkane, partially hydrolyzed polyvinyl acetals and polyvinyl alcohol esters, amylose and regenerated cellulose. In addition, other plastics such as suitably prepared polyamides or nylon-type plastics may be employed for carrier materials.

Of the plastics herein named, polyvinyl alcohol is preferred.

To make long chain plastics of this character light-polarizing, their molecules must be substantially oriented. Orientation may be effected;

by stretching plastic sheet until the molecules thereof are sufiiciently oriented. Aglternately, the surface molecules of one or both sides are plastic sheet may be oriented as by the application of linear, frictional forces thereto without.

4 orienting the molecules throughout the remainder of the sheet.

Of the carrier materials heretofore set forth, the invention, as intimated, intends to use orientable plastics in the formation of both light-polarizing images and nonpolarizing images. At the same time the invention comprehends' the use of hydrophylic materials other than orientable plastics, gelatin for one example, in the formation of nonpolarizing images by novel methods which will presently be described.

A support is generally employed for the carrier or carriers of the photographic films of the invention. The support may be a plastic material although it may be glass and in certain instances paper which may have a metallic or other reflecting coating provided theron. In fact any of the films to be later described may be superposed on a light-reflecting backing, at least after image formation, to provide a reflection print.

Examples of suitable transparent support materials comprise a cellulosic plastic, for example cellulose esters such as cellulose acetate and cellulose nitrate or cellulose mixed esters such as cellulose acetate butyrate or cellulose acetate propionate, Or a vinyl compound, such as the vinyl acetate-chloride copolymers, or a suitable condensation type superpolymer, such as polyamide or nylon-type plastic. The various types of cellulose esters such as cellulose nitrate and cellulose acetate butyrate may be named as preferred transparent materials for the support. It is also to be understood that such materials as polyvinyl alcohol and regenerated cellulose may be used to provide a support.

The different layers of the films with which the invention may be practiced may be coated on or laminated to one another. When an oriented carrier is employed, orientation may be effected after formation of the film, or in the case of the use of several carriers, they may be oriented before or after they are laminated.

The practice of the invention may be most simply understood with reference to the formation of light-polarizing images in a film Ill like that forming the subject of illustration, modifications of the invention becoming apparent to those skilled in the art without further detailed illustration when considered in the light of the processing techniques employed for such film together with the explanation of such modifications.

The film l0 shown in the drawing comprises a base or support [2 on which is supported a carrier or an image-bearing layer l4 formed of a transparent, hydrophilic, molecularly oriented polymeric plastic such as polyvinyl alcohol, the oriented molecules of the layer being schematically illustrated by the dash lines It. Sensitization of the carrier M. of film It) provides a light sensitive element suitable for the practice of the invention. Conventional silver halide forming solutions may be imbibed in the carrier 14 to effect sensitization. Also, a solution of a plastic material of the character mentioned having silver halide dispersed therein may be cast into a carrier which may then be subjected to processing such as that described to eifect orientation of the molecules of the plastic.

A silver image may be formed in the film l0 sensitized as just described by suitable exposure and development in conventional manner. The silverimage is then transformed to an iodine image l8, the first; step of the processing being to convert the silver image to an image formed of silver ferrocyanide as by bleaching or processing with a suitable alkali ferricyanide. The silver ferrocyanide image is in turn transformed to an image comprising a mixture of water-insol ublemetallic ferrocyanides which do not include silver ferrocyanide. At least one of the ferrocyanides of the mixture can be converted to its corresponding hydroxide to provide an oxidizing agent having a sufiiciently high oxidation potential to release iodine from an acid solution of an iodide.

All of the metallic ferrocyanldes in the mixture may be transformed to hydroxides on suitable treatment. In such case at least one of the hydroxides which are formed lacks the-ability to serve as a potential oxidizing agent capable of releasing iodine from an acidic iodide solution. On the other hand certain ferrocyanides of a univalent metallic element or a polyvalent metallic element in lower valent form are soluble in the reagent used for hydroxide conversion. For example, zinc ferrocyanide will be dissolved in a sodium hydroxide solution which is a preferred treating solution for the image formed of the mixed ferrocyanide's. However, as will presently appear, the removal of any ferrocyanide of a univalent or polyvalent metallic element in lower valent form can take place at any stage of the processing subsequent to its formation without efiect on the final result to be attained.

Film it, after formation of the image comprising the oxidizing agent, is treated in an acid solution of an iodide, there-by releasing iodine from the iodide. As any hydroxides which form the image are soluble in acid, the hydroxide image is effectively removed and replaced by the released iodine. As a result of this transformation the iodine is physically located in the same portions of the film as the originally exposed silver halide particles and forms the desired dichroic or light-polarizing image 18 in the molecularly oriented carrier l2.

The principles underlying the invention may be understood when it is considered-that certain metallic salts may be employed to tone silver images which have been bleached to silver ferrocyanide. Notable examples are iron and copper salts employed to tone a silver image bleached for example with potassium ferricyanide to an image of silver ferrocyanide. Toning of a silver ierrocyanide image with an iron or a copper salt in the just-described manner will result in the formation of an image of Water-insoluble iron or copper ferrocyanide.

Any metallic salt which forms an insoluble ferrocyanide upon reaction with silver ferrocyam'de will serve in the processing of a silver image bleached in this manner. If the processing agent is a salt of a polyvalent metallic element in higher yalent form, this higher valent metallic ion will be capable of releasing iodine from an acid solution of an iodide upon conversion of the ferrocyanide which is formed by such metallic salt to its corresponding hydroxide. On the other hand if the processing agent is a salt of a univalent element or of a polyvalent metallic element in lower valent form, the metallic ion will be incapable of releasing iodine from acid solutions of iodides. Hydroxides thus formed are soluble in acid solutions and hence are removed when the film is treated with an iodide-containing solution of acidic'character.

The processing solutions of this invention com- :prise a-mixture of at least one salt of a poly.-

valent metallic element in higher valent form with atleast one salt of a univalent metallic element other than silver or of a polyvalent metallic element in a. lower valent form. The salts are selected to provide insoluble metallic ferrocyanides upon toning of silver images bleached iodide. I

From this it follows that by controlling the proportions of the two types of metallic salts used to produce the image of mixed ferrocyanides, the proportions of the ierrocyanides in the mixture can be selected and the concentration of ions which can be made available to release iodine from a given acid solution of an iodide can be controlled by the presence in the toning solution of the salt of aunivalent metallic element or a polyvalent metallic element in lower valent form. As it is the oxidizing agent which causes the release of iodine, control of the ion concentration.

of the polyvalent ions in higher valent form resulting from the quantity of oxidizing agent present will therefore determine the effectiveness of the oxidizing agent and the quantity of iodine releasedirom a given iodide solution andhence the density of the iodine image.

Metallic salts which may be employed as the component providing ions which will subsequently secure the release of iodine comprise salts of polyvalent metallic elements in higher valent forms, such for example, as cupric or ferric salts. Metallic salts which may be employed as the component for modifying the density of the final iodine image through conversion of a portion of r the original silver image into a material incapable of releasing iodine comprise salts of such univalent metallic elements as zinc or cadmium, and such polyvalent elements as nickel, uranium, vanadium, or cobalt in their lower valent forms.

In the process, as heretofore set forth, the transformation of silver to silver ferrocyanide and the conversion of silver ferrocyanide to the mixture of ferrocyanide salts of metallic elements have been indicated as two distinct operations which may be carried out in separate solutions. However, conversion of silver to the mixed metallic ferrocyanides may be carried out more rapidly by effecting the transformation in a single solu-- tion containing all the reactants, that is to say, the fe'rricyanide and the metallic salts. If the reactions are carried out in a single solution, precautions are taken to prevent direct reaction of the metallic salts with the ferricyanide adapted to form the silver ferrocyanide. This may be accomplished bythe addition to the solution of a suitable composition comprising an organic salt, such as ammonium oxalate, and an acid, such as oxalic acid. Other materials which may be used instead of ammonium oxalate for preventing undesired interaction between the components of the solution are, for example, potassium oxalate, potassium citrate, ammonium citrate, potassiumsodium tartrate, ammoniumtartrate, sodium succinate and other organic salts. Instead of oxalic 3,Cldj. 0th8]l acids, such'as hydrochloric acid and weak .alkalies, for example, sodium carbonate,

potassium carbonate and ammonium carbonate, may be used.

, The novel features of this invention will more fully appear from a detailed descriptionof a typical exampleof the formation of black-and-white vectograph prints in which the photographic image is a dichroic iodine image.

Silver halide-sensitized, molecularly oriented polyvinyl alcohol film is exposed to the subject to be reproduced, developed, stopped, and Washed in the usual manner, employing standard procedures therefor. 'Ihe film with at least one silver image therein is then treated with the following solution, comprising bleaching and toning agents which are mixed in the order given below.

Cubic centimeters Ammonium oxalate, aqueous solution '70 Nickel nitrate, 10% aqueous solution 10 Cupric sulfate, 10% aqueous solution 25 Potassium ferricyanide, 10%, aqueous solution l2 Oxalic acid, 14% aqueous solution 4 A three minute treatment in the above solution is ordinarily sufficient to bleach and process or tone the usual silver image. However, the time may be varied depending upon the density of the silver image obtained or on the qualities of the particular polyvinyl alcohol-silver halide emulsion employed. The processed film is then washed for two minutes. As the next step, the cupric and nickel ferrocyanides comprising the image are simultaneously converted to their respective hydroxides by treatment for example in an aqueous solution containing 10 sodium 'thiocyanate, 10% sodium thiosulfate, and 1% sodium hydroxide. This; solution also fixes silver halide remaining in the carrier. Such practice is preferred from the standpoint of simplicity although if desired the carrier may be fixed by separate treatment and then subjected to a sodium hydroxide solution to effect formation of the oxidizing agent. It is to be noted that in the example given there is employed a control salt, that is nickel nitrate, which will not be dissolved in the formation of the oxidizing agent used to release iodine but instead will be converted to its hydroxide. Treatment to form the oxidizing agent which will release iodine is continued for about 1 minute after which the film is washed for about 3 minutes and is then treated with an acid iodide solution to effect the formation of the iodine image.

A suitable iodide solution comprises:

Cubic centimeters n The product of this immersion contains an image in iodine. It is preferable to immerse the print in a final stabilizing bath, which bath contains boric acid to stabilize the dichroio image. The stabilizing bath mayalso contain sodium sulfate to prevent softening of the sheet, potassium iodide to provide a control for the color of the image and in some instances .a weak solution of sodium thiosulfate.

While the inventionhas been described in connection with the formation of a light-polarizing image or vectograph image in a film having a single image carrier of molecularly oriented plastic material, it will be understood that a film comprising two such layers, either on the same side of the support or on opposite sides of the support,

.may have one or more pairs of such images formed therein. The images may be of the same subject and may bear a predetermined relation to one another. For example, one may be the left-eye image and the other the right-eye image of a stereoscopic pair and the directions of molecular orientation of the plastic carriers for such images may be at substantially right angles to each other so that an observer of the iodine images formed in the carriers may view each pair of images stereoscopically with the aid of suitable light-polarizing viewers.

It will likewise be understood, to those skilled in the art, that light-polarizing images may be formed in a film similar to that of film Ill wherein the molecularly oriented, layer supports a layer of gelatin in which the iodine image is formed, the iodine migrating from the gelatin into the oriented plastic. Similarly, the practices heretofore and subsequently described are adapted for the formation of nonpolarizing images by the formation of iodine images in unorientable material, such as gelatin, or in orientable material, such as polyvinyl alcohol, the molecules of which are in an unoriented condition.

The foregoing practices can be readily adapted for transforming the silver image of a conventional photographic paper print to an iodine image, which iodine image can then be transferred to a sheet comprising at least a surfacelayer of molecularly oriented polyvinyl alcohol. In a preferred adaptation of the transfer process, the iodine is not released until the paper print is brought into intimate contact with the polyvinyl alcohol surface.

This modification of the process comprises transforming the silver image in the paper print to an image containing a hydroxide of a polyvalent metal in higher valent form, as previously described above, and then immersing the print in a solution of an iodide which does not contain an. acid. This brings the iodide into intimate contact with the hydroxide image but because of the absence of the acid does not release iodine. Acid for rendering the metallic hydroxide capable of elfecting the release of the free iodine from the iodide solution is contained in the oriented polyvinyl alcohol surface with which the print is then brought into surface contact. The iodine is thus simultaneously released and transferred to the polyvinyl alcohol surface. The acid, as for example maleic acid, may be introduced into the polyvinyl alcohol surface in any Well known manner, as by casting the polyvinyl alcohol layer or sheet with the acid contained in the casting solution. When this transfer method of obtaining dichroic images is used, it is unnecessary to wash out the silver salts after the metallic ferrocyanide image is formed.

In effecting the modified processing just described, the conventional paper print for a silver image formed therein is immersed in a treating solution such as that disclosed'for obtaining the image formed of the mixture of water-insoluble ferrocyanides. Immersion in the solution for about two minutes gives satisfactory results. The

paper print is then immersed in the sodium hy- 'droxide bath, either with or without being first washed in sodium thiosulfate to remove silver salts. After this the print is dipped in a 10% potassium iodide solution which contains no acid. Following this, the image carrier comprising a molecularly oriented sheet of a plastic material, such as polyvinyl alcohol, and containing approximately 10% by weight of maleic acid, is brought 'into contact with the print whereby iodine is released and is transferred to the plastic carrier to give the desired light-polarizing image.

The film materials described maybe employed as out, roll or motion picture film. They may be used as positive or negative forming materials but are especially adapted for positive printing purposes from prepared negatives. The products obtained by processin any of the photographic elements disclosed may be used as transparencies or they may be mounted upon a reflecting backing sheet, such as metallized paper and viewed as refiection prints. It is to be kept in mind that the processing techniques and carrier materials emloyed in the practice of the invention may be utilized to provide an iodine image in each carrier layer of a film and that such images may be nonpolarizing or light-polarizing in character. Furthermore, the invention is especially adapted for the formation of images in the nature of stereoscopic pairs whereby to permit stereoscopic rendition or representation.

It should be pointed out that although the foregoing description emphasizes the formation of iodine images which are duplicates of photographic silver images, the invention is in no way limited to the exclusive use of photographically obtained silver images.

It is also to be understood that the terms image in iodine or iodine image as used in the claims refer to an image formed of iodine or its complexes.

From the foregoing it will be appreciated that the aims and objects of the invention have been accomplished in that novel processes have been provided whereby iodine images of predetermined density may be formed in photographic carrier materials. Likewise, in attaining the aims and objects of the invention, there has been provided a product which will predeterminedly effect the release of iodine from an acid solution of an iodide whereby to form an iodine image of selected density.

Since certain changes in carryin out the above process, and certain modifications in the product which embody the invention may be made Without departing from its scope, it is intended that all matter contained in the above description, or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of forming an iodine image of predetermined density from a silver image which is supported'by a hydrophilic carrier, comprising treating the silver image to replace the silver thereof with a mixture of water-insolublemetallic ierrocyanides other than silver and comprising a predetermined quantity of at least one metallic ferrocyanide which may be formed into an oxidizing agent having a suificient oxidization potential to release a predetermined quantity of iodine from an acid solution of an iodide and at least one other metallic ferrocyanide which is incapable of forming an oxidizing agent possessing the ability to release iodine from an acid solution of an iodide by subjecting said silver image to the action of an alkali ferricyanide and a mixture of metallic salts formed of a predetermined quantity of at least one metallic salt selected from the class consisting of cupric salts and ferric salts and at least one other metallic salt from the class consisting of zinc salts and cadmium salts and of salts of nickel, uranium, vanadium and cobalt in which the metallic element is in lower valent form, transforming water-insoluble metallic ferrocyanide having-a metallic. element from said first-named class into the desired oxidizing agent for efiecting the release of iodine by subjecting said mixture of water-insoluble metallic ferrocyanides to the action of a base, and reacting said oxidizing agent so formed with said acid iodide solution whereby iodine in accordance with the quantity of oxidizing agent present'is released and replaces the oxidizing agent to form an iodine image of desired density.

2. A method of forming an iodine image of predetermined density from a silver image which is supported in a hydrophilic carrier, comprising replacing the silver image with a mixture of water-insoluble metallic ferrocyanides other than silver ferrocyanide by treating the silver image with an alkali ferricyanide and a mixture of metallic salts in predetermined proportions to each other and in quantity sufficient to react with substantially all silver ferrocyanide formed by the reaction between said alkali ferricyanide and said silver, said mixture of metallic salts comprising at least one metallic salt from the class consisting of cupric salts and ferric salts whereby to provide at least one water-insoluble metallic ferrocyanide capable of conversion to an oxidizing agent possessing the ability to effect the release of iodine from an acid solution of an iodide in accordance with the quantity of oxidizing agent present and also at least one metallic salt from the class consisting of zinc salts and cadmium salts and of salts of nickel, uranium, vanadium and cobalt in which the metallic element is in lower valent form whereby to provide at least one water-insoluble metallic ferrocyanide incapable of conversion to an oxidizing agent for efiecting the release of iodine from acid iodide solution, converting metallic ierrocyanide which has a metallic element from said first-named class to its correspondin hydroxide whereby to provide the desired oxidizingagent for releasing iodine by treating the mixture of water-insoluble ierrocyanides with a base and reacting said metallic hydroxide with said acid iodide solution whereby iodine in accordance with the quantity of said hydroxide present is released and replaces the hydroxide to form an iodine image of desired density.

3. In a method of forming an iodine image of predetermined density from a silver image in a hydrophilic carrier, the steps of treating said silver image with a solution comprising potassium ferricyanide, nickel nitrate and cupric sulfate in predetermined proportions to each other whereby to convert said silver to a mixture of cupric and .nickel ferrocyanides, subjecting said mixed ferrocyanides to the action of sodium hydroxide whereby to convert the mixed ferrocyanides to their correspondin hydroxides, and then causing the cupric hydroxide to release iodine in proportion to its quantity present in the carrier by treating the carrier with an acid solution of an iodide.

4. The method of forming a dichroic image of predetermined density, comprising forming an image in silver in a relatively water-insoluble carrier having at least a layer of a permeable, molecularly oriented, high polymer possessing an aiilnity for iodine and adapted to form a dichroic sorption complex therewith, said layer supporting said silver image, transforming the silver from said silver image to a mixture of water-insoluble metallic ferrocyanides other than silver and comprising a predetermined quantity of at least one metallic ferrocyanide which may be formed into an oxidizing agent having 'a -sufficient oxidization potential to release a--predeter mined quantity of iodine from an acid solution of an iodide and at least one other metallic ferrocyanide which is incapable of forming an oxidizing agent possessing the ability to release iodine from an acid solution of an iodide by subjecting said silver image to the action of an alkali ferrocyanide and a mixture of metallic salts formed of a predetermined quantity of at least one metallic salt'selected from the class consisting of cupric salts and ferric salts and at least one other metallic salt from the class consisting of zinc salts and cadmium salts and of salts of nickel, uranium and vanadium and cobalt in which the metallic element is in lower valent form, transforming Water-insoluble metallic ferrocyanide having a metallic element from said first-named class into the desired oxidizing agent for effecting the release of iodine by subjecting said mixture of water-insoluble metallic ferrocyanides to the action of a base, and reacting said oxidizin agent so formed with said acid iodide solution whereby iodine in accordance with the quantity of oxidizing agent present is released and replaces the oxidizing agent to form an iodine image of desired density.

5. The method of producing a light-polarizing image of predetermined density in iodine, which comprises forming the image in silver in a permeable, relatively water-insoluble carrier, treating the silver image to replace the silver thereof with a mixture of water-insoluble metallic ferrocyahides other than silver and comprising a predetermined quantity of at least one metallic ferrocyanide which may be formed into an oxidizing agent having a suflicient oxidization potential to release a predetermined quantity of iodine from an acid solution of, an iodide and at least one other metallic ferrocyanide which is incapable of forming an oxidizing agent possessing the ability to release iodine froman acid solution of an iodide by subjecting said silver image to the action of an alkali ferrocyanide and a mixture of metallic salts formed of a predetermined quantity of at least one metallic salt selected from the class consisting of cupric salts and ferric salts and at least one other metallic salt from the class consisting of zinc salts and cadmium salts and of salts of nickel, uranium, vanadium and cobalt 7 iodine is released in accordance with the quantity of oxidizing agent present and replaces the oxidizing agent to form an image in iodine of predetermined density, and bringing into face-toface contact with said carrier a transparent, molecularly oriented high polymer adapted to form a dichroic sorption complex with iodine and having a greater afiinity for iodine than said carrier. HELEN P. HUSEK.

REFERENCES CITED The following references are of record in the file of this patentz' UNITED STATES PATENTS Date Number Name 1,009,100 Rehlander 1- NOV. 21, 1911 1,389,742 Crabtree Sept. 6, 1921 7 1,957,420 Butez Mayl, 1934 2,019,718 A Mannes Nov. 5, 1935 2,099,297 Clement NOV. 16, 1937 2,176,585 Evans Oct. 17, 1939 2,183,661 Trimble Dec. 19, 1939 2,209,548 vl'hitlock July 30, 1940 2,226,339 Crespinel Dec. 24, 1940 2,315,373 Land Mar. 30, 1943 2,316,644 Yule Apr. 13, 1943 2,341,079 Bunting '1 Feb. 8, 1944 2,346,775 Mahler Apr. 18, 1944 2,365,416' Kuhne Dec. '19, 1944 2,409,923 Barnes Oct, 22, 1946 FOREIGN PATENTS I Number Country Date 469,133 Great Britain July 20, 1937 OTHER REFERENCES 319, cited. Published by Longmans Green and

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