Popular Science, tammikuu 1883
By Captain Abney, R. E., F. R. S.
Taking the case of a daguerrotype plate which has been exposed, and which we are about to develop by the action of mercury, I should like you to understand exactly what takes place in this plate when it is exposed and developed. On the surface of the plate we have a mixture of silver iodide and bromide; but, for simplicity's sake, I will suppose that it is simply silver iodide. When light acts on such a compound, the result is the liberation of iodine and the formation of a new salt, which we call silver subiodide, Ag2I2 = Ag2I+I. The idone is taken up by the silver plate at the back of the sensitive film. To develop the picture, mercury.vapor is caused to condense on the subiodide, and leave the iodide intact. In the Talbotype process, the picture, which has been taken on a paper that has been washed with nitrate of silver, iodide of potassium, and nitrate of silver again, is developed by washing with gallic acid and silver nitrate. The picture begins to appear on washing after a very short exposure to the light, and becomes gradually more visible as the washing goes on. A paper process is a most fascinating process, because you can dabble about, and do exactly what you like; it is not like the gelatine plates of the present day, which you have to leave to come out mechanically. With paper, if you want to bring out a little better detain in one place, you can dab it out, and, if you want to keep it back, you can put a little water over the place. There is no process like the paper process to please an artist. Now, what is the meaning of the development in this process? This morning I was in my laboratory, and I saw lying on the bench a feeble negatice which I had badly developed, and which I had fixed with hyposulphite of soda. On taking it up, I found the salt had crystallized over the surface in the most beautiful manner; and I do not think I could point out to you anything which would give you a better idea of what development is than those crystals. When you have silver precipitated from a solution by any means whatever, you have it always in a crystalline form, and, as all crystals possess polarity, so crystals of silver possess polarity; and where one silver particle is deposited, there another silver particle will deposit. I look upon this as a physical development; we have a crystalline action going on during development, and nothing else. The iodide of silver is altered into a subiodide, and this, like the pole of magnet, attracts the precipitating silver, and from that time, where the silver is deposited, other crystals of silver are deposited. That is what I call physical development.
There is another kind of development which some call chemical development; it is shown by a change in the color or material of the substance acted upon, and not by a building-up process, such as we have just had illustrated. The process may be illustrated in the development, by means of silver nitrate, of a picture which has been printed on nitrate of uranium. The picture is formed by silver oxide reduced by the particles of uranium nitrate which have been acted upon by light, and by nothing else. the silvver oxide reduced is an exact equivalent of the uranium salt which has been acted upon by light. This differs from the previous process in that the gallic acid, in the one case, reduces the silver solution to the state of metallic silver; and, in the other case, the uranious image itself reduces it to the state of silver oxide.
Another mode of development, called chemical development in Germany, may, I think, more properly be termed alkaline development. Its theory is, that when you have a strongly oxidizing agent in the presence of an alkali and a silver compound, solid or in solution, the last will be reduced to the metallic state. Such an oxidizing agent we have in pyrogallic acid, and the alkali generally used in ammonia. Now, this kind of reduction is evidently useless, unless it can discriminate between a compound which has been acted upon by light and one which has not. When pyrogallic acid is used, in order to make the discrimination, something more has to be added as a restrainer to cause the reduction including the change to take place only in the part acted upon by the light. A solution of the bromide of an alkali is generally used for this purpose. Without a restrainer, the tendency is for those parts to be first reduced, but the action extends to that which has not been acted upon by the light. It has been usually said that alkaline development is only available for bromide of silver, but my experience has taught me that iodide of silver is as amenable to alkaline development as the bromide, although not so rapidly, and that chloride is very amenable to it, and will give most beautiful pictures.
Another mode of development, now very much in vogue, is that with ferrous oxalate. In this case we have an organic salt of iron in the ferrous state, which is capable of reducing silver bromide, iodide, and chloride to the metallic state, while itself is reduced to the ferric state. This process also requires a restrainer.
I have found a kindred developer, the use of which I consider one of the most recent advantages in photography. It is an iron developer, which is capable of being used without any restrainer whatever. I call it ferrous citro-oxalate. It is made by adding to a solution of citrate of potash ferrous oxalate till no more will dissolve; the resulting compound is probably citrate of iron, ut in a stronger form than is usually found.
Mr. Berkeley has lately introduced an improvement in the ordinary alkaline developer, in which he mixes with the pyrogallic-acid solution four times the weight of sulphite of soda. The action, apparently, is that the sulphite of soda absorbs oxygen with greater avidity than does the pyrogallic acid, thus leaving that agent to do its work; consequently, we have a developer which remains uncolored for a very long period.
Another developer, which is competent to work also without a restrainer, but has not been used to a very great extent on account of its high price, is hydro-kinone. It is a much more powerful absorber of oxygen than pyrogallic acid, to such a degree that one grain of it is as active as two grains of that substance. Not requiring any restrainer, even when so troublesome a salt as silver chloride is used, it is able to give a better detail and allow a shorter exposure in the camera than when the ordinary alkaline developer is used. It is applicaple to any plate with which you can work.
The next point to which I wish to call attention is the action of sensitizers. It may be proper first to explain what a sensitizer is. When you have chloride of silver exposed to light, you have a new compound formed, which is called subchloride, or argentous chloride (Ag2Cl2 = Ag2Cl+Cl), and chlorine is liberated. This chlorine is very difficult to eliminate, if you do not give it something that can take it up; for instance, if you place perfectly pure chloride of silver in vacuo, without any trace of organic matter present, you will find that you get no darkening action, even if it is exposed to brilliant sunlight for months. If a white powder of that kind was submitted to you, to determine its character, you would say at once that it was not chloride of silver, because it was not darkened, since one of the tests of chloride of silver, among chemists, is that it shall darken in the light. Here I have a little bulb of it which was prepared, dried carefully, and sealed up. It hs been exposed for months to the light, and is as pure a white as it was the first day it was put into the bulb. Another experiment was made at the same time; but, unfortunately, as I thought then, a small globule of mercury got into the vacuum, and was sealed up with the chloride; the consequence was that the chloride of silver immediately darkened: although the mercury was not in contact with the salt, the chlorine flew to the mercury, and formed chloride of mercury. This is an instructive experiment, showing that chloride of silver will darken merely in the presence of something that will mop up the chlorine. Silver idodide, when exposed to light splits up into silver subiodide and iodine, and silver bromide into silver subbromide and bromine. Now, in order that there shall be a ready darkening of either of these salts, you must have something which will absorb the iodine or bromine (or, in the case of the latter, allow it to escape), according to the salt you expose to the light. This something is the sensitizer.
One point that has exercised the minds of a great many photographers is the illumination of their dark rooms. [The lecturer having shown the relation of the several parts of the solar spectrum with the absorption properties of different substances used in photography, proceeded to demonstrate the effect of differently colored glasses upon the passage of rays, and announced his conclusions.] If photographers want to have an absolutely safe light in developing their pictures, let them glaze their studios with cobalt glass and stained red, and they will get nothing but the light of that particular refrangibility, which will not affect any gelatine plate of the ordinary type. You may glaze and glaze with ruby, but you will never get rid of blue light entirely. Of course, it diminishes with every thickness you take. If you want to use ordinary plates, which are not so sensitive that you can not look at them, my advice is to use a combination of stained red and ruby glass, which will give you a comfortable light to work in, for it cuts off the blue and leaves the red in a brilliant patch. If the operator wishes to be still more secure, let him use a combination of cobalt glass and stained-red glass. A combination of red and green is a fairly safe light for iodide plates or ordinary plates, but not for gelatine plates, which are very sensitive. Next we come to a series of pretty colors, which may be very useful to us: magenta, with which the yellow is cut out entirely, and the green, leaving the blue, violet and orange; aurine and chrysoidine, which cut off the blue; a combination of magenta and aurine, which gives a perfect red light, and is very good indeed for the photographic studio; and scarlet and aurine, which give the same effect. If all means of securing the right light fail, the photographer may use the ferrous exalate developer, for you may bring the most sensitive plate out into a white light, when developing in a dish with a covering of that substance over it.
In 1874 the discovery was made that an increased action of the spectrum could be got by dyeing the film of sensitive collodion. If you take one of the aniline dyes and expose it to the light behind a piece of black paper, you gen an image on the dye. What is the meaning of that? The meaning is, that the dye is oxidized, for, if you apply an oxidizing agent, you get the same result. Dr. Vogel found that if he dyed a plate with one of these fugitive dyes, he was able to obtain an extension of the impressed spectrum, and he introduced the term "optical sensitizer" to describe the fact. I object to the term, for it gives a wrong impression of the action that takes place, which is simply the reduction of the iodide or bromide of silver by the oxidation of the dye, and the provision of a nucleus on which development can take place.
Collodion emulsions have been in vogue for seven or eight years, although they have now been superseded, to a large extent, by gelatine emulsions. Whether the last be an improvement over the former process or not, the collodion process is admirably adapted for landscape-work. If the emulsion is of silver bromide or chloride, it is easily formed; an iodide emulsion is more difficult. The point in emulsion-making seems to be to get the precipitate in as fine particles as possible, and it is said that this can only be obtained, except at very great cost of time and trouble, by first adding the soluble bromide or iodide to the collodion. If you take the trouble to add the silver to the collodion first of all, the aspect of emulsion-making is entirely changed, and you can get any amount of fineness by adding the iodide or bromide to the silver contained in the collodion so long as you keep the silver nitrate in excess. If you put the iodide into the collodion first, and then add silver nitrate, you will find that you have precipitated the iodide of silver at the bottom of the bottle, and in a form which will not emulsify at all. My advice to those who wish to make collodion or gelatine emulsion is, to add the silver to the collodion or gelatine, and then add the haloid salts afterward, and you will get as perfect an emulsion as you choose.
It is a great comfort in the collodio-bromide process that the operator is able to give local intensity (a most desirable quality in all artistic work) to image. I do not believe any process is perfect until that power is placed in the hands of the manipulator; and the great defect of the next process to be mentioned is, that it does not give that power, but leaves the operator at the mercy of his plate, on which he must let come out what will. This next process is the gelatine process, which may be described as one in which the silver bromide is held in suspension in gelatine in the same way that in the previous process it is held in collodion. Mr. Bennet showed how a gelatine emulsion can be made very sensitive by keeping it at a comparatively low temperature in a liquid condition for many days. Colonel Wortley afterward claimed that he could get the same sensitiveness by heating up to 150° Fahr. for a short time; and then Mr. Mansfield got it in a few minutes by boiling. Another method was then introduced by Dr. Monkhoven for the production of very sensitive gelatine emulsions by adding ammonia with the nitrate of silver. The ammonia process found many admirers, among them Dr. Eder, whose method of adding a large quantity of ammonia has given very sensitive pictures, and very vigorous ones when the sensitiveness is not too great. A process introduced by Mr. Cowan is even superior to that of Dr. Eder. He emulsifies his bromide in a very small quantity of gelatine with ammonia, and adds sufficient gelatine when the emulsion is ripened. Dr. Eder's method was to add the full amount of gelatine with the ammonia. Mr. Cowan's method gives greater rapidity and greater certainty.
What is the reason of the sensitiveness of the gelatine emulsion? Pictures can be taken with it in a tenth of the time necessary for a wet plate, and perhaps a thousandth of that necessary for an ordinary dry plate. The first reason is, that the emulsion has a blue form. Another reason is, that you can use a more powerful developer. If you separate bromide of silver which has been emulsified in gelatine, and place it in collodion, the extreme rapidity will be gone, for the simple reason that you can not use as strong a developer as you can with a gelatine emulsion; in fact, the property that gelatine possesses of acting as a physical restrainer comes into play: each little particle or aggregation of particles of the salt is surrounded by gelatine, which prevents the developer acting rapidly on them. Again, the fact that by boiling, or by the ammonia process, you get a coarser deposit of bromide of silver, also points to increased sensitiveness. Furthermore, if you boil or heat bromide, or any haloid salt or silver, with an organic substance, it has a tendency to separate into a metallic state; in fact, the bromide of silver is then in a state of very tottering equilibrium; the bromine is ready to be given off at the very slightest disturbance of the molecule, much more so than before it is boiled. I think that the fact that you so often get fogged emulsion when you overboil is proof of this statement. If you were to ask me to illustrate the sensitiveness of a gelatine plate, I should show you, not some of those marvelous instantaneous photographs, but a photograph by Mr. Henderson, by moonlight, and another of some under-ground cellars at Reigate, by Mr. William Brooks, taken by lamp-light. If anything can show what gelatine plates can do, it is the fact that candle-light and moonlight can be utilized for impressing the surface with an image. Dr. Vogel has recently introduced an emulsion made with acetic acid, gelatine, pyroxyline, and bromide of silver, which is very clean and very fairly rapid. Plates are more readily coated with it than with gelatine emulsion, but less so than with collodion emulsion.
Anoher very decided advance in photography is the doing away with glass as a support for the emulsion. Mr. warnerke has perfected a process by which the photograph is taken on paper instead of on glass. He has a sensitive tissue which can be made of any length, and can be rolled on a roller and exposed in the dark slide. By turning another roller, a fresh surface is brought into the plane of the focusing-screen. The sensitive tissue is developed in the ordinary way with alkaline development. The film can be either stripped off, or else transferred to glass. In the latter case, we come to another point which marks a distinct advance. Mr. Warnerke has found that when you develop a gelatine plate with alkaline development, the parts which have been acted upon by light, and which have been developed, become insoluble in hot water. He is thus able, after development, instead of using the hyposulphite bath to fix the print, to transfer it to glass, and was away with hot water the parts of the film which have not been acted upon by the light; and he thus gets a transparency. To do this, it is necessary that the back surface of the gelatine film should be exposed to the water, as in carbon printing, and this is secured by transfer to glass. Mr. Warnerke is not satisfied with doing away with glass for the camera, but he does away with glass for printing; and, in order to accomplish this, he retransfers the negative from the glass to a sheet of gelatine. I may say that the glass is freshly collodionized, and this enables the film to strip off readily. It is one of the advantages of these negatives that you can print from either side, each one yielding sharp points - a desideratum when using processes where reversed negatives are required. In the matter of gelatine films, we have Professor Stebbings's, which are really workable. The gelatine emulsion is apparently flowed on an insoluble film on glass, which is then stripped.
The next point I touch upon is the enlargement of negatives. The best way I know of, of getting an enlargement of a negative, is one that was brought forward a few years ago by Mr. Valentine Blanchard. He takes the original negative which he wishes to englarge, and places it in an enlarging camera. He then takes a transparency of the exact size he wants his negative to be. He next takes a piece of common albumenized paper, and prints that transparency upon it, and by this mean gets a very soft an beautiful negative. If you have a hard negative, it is almost impossible to get a soft transparency by the wetplate process; but, by this artifice of "printing out" your transparency and using that as a negative, you get a decidedly soft paper negative.
One of the new applications of the gelatine process is the development of a print on paper coated with gelatino-bromide. The paper is prepared by coating ordinary paper with gelatino-bromide (of the most sensitive kind, if you like). Such paper can then be exposed to the image formed by an ordinary magic-lantern; by that means you can get an enlarged print. We may thus say that an advance has been made, when, by an ordinary magic-lantern, with a good negative, you can get a perfect enlarged print by development. Perhaps it will not have that luster which albumenized prints have, but it is a matter of taste whether you like that gloss or not.
As gelatine plates are now prepared they all have an excess of soluble bromide. While this is the case, the highest sensitiveness possible will not have been obtained. Dr. Eder has found that an increase of sensitveness, two or three fold, may be produced by neutralizing this excess. The gelatine-plate makers have the problem to solve, how to get rid of any possible excess of soluble bromide in their films.
We will next consider what causes the destruction of the photographic image. You may destroy it by any substance which will readily part with oxygen. You can destroy it by any substance which will readily part with oxygen. You can destroy it, for instance, by bichromate of potash, by any of the ferric salts, or by oxygen-yielding substances, like permangate of potash, ozone, peroxide of hydrogen, or hydroxyl; in fact, there is hardly any substance which will part with oxygen, which will not destroy the developable image. The photographic image remains behind as a rule, though not always, but these re-agents prevent it becoming developable. Bromine also acts sometimes as a destructive agent, by escaping, when the exposure is too long, from the lower part of the bromide coating of the plate, and forming a fresh film of bromide at the surface after it has been acted on by the light.
A remarkable utilization of the oxidizing process has been proposed and carried out by M. Bolas. Wishing to reproduce an ordinary gelatine negative having the proper gradations of light and shade, he took a gelatine plate, immersed it in bichromate of potash, allowed the film to dry, and then exposed it to light behind the negative to be reproduced. In this exposure he had an oxidizing agent present in his film; the oxidized parts were acted upon by the light, leaving the other part intact; and by that means he got a reversed image. Oxidizing agents enable us also to get rid of fog. A gelatine plate, which has been fogged by exposure to light, can be cleared by immersing it n bichromate of potash.
I have learned in my experiments that halations, or the appearance of haloes around the picture can be prevented, by touching the back of the plate with asphaltum or some varnish; the reflection is toned down according to what medium is placed on the back of the plate. The most perfect cure for halation is Brunswick-black. It admits no reflection from the back of the plate, and thus enables the operator to get rid of every tendency to fuzziness of the image.
A most useful instrument has been introduced by Mr. Warnerke, which is known as a sensitometer, or measurer of sensitiveness. It consists of squares of colored gelatine of different opacities through which light is allowed to fall on a sensitive plate, and is intended as a guide to determine the comparative rapidity of the plates. Mr. Warnerke has also introduced an actinometer, or instrument to measure the intensity of light, which is dependent on phosphorescence for its value. It consists of a phosphorescent tablet, by the exposure of which to the action of light he is able to tell the photographic value of the particular light. The discovery is of the more value, because phosphorescence is induced by very nearly the same rays as those which affect bromide of silver. Another silmple way of telling the amount of exposure to give the plates is by Woodbury's photometer, in which a piece of bromide paper exposed to the light is compared and read off with one of a series of tinted circles. A rule to be remembered in using this instrument is, that if a bromide plate is used, a bromide paper only should be used for securing the tint; if a chloride plate, a chloride paper. Recent researches of mine have shown that the darkening intensity and the developing intensity go hand in hand; therefore, when the operator has the number which gives the right tint, he may always be sure of getting the right exposure.
Some of the most recent and striking exemplifications of the scientific applications of photography are the composite photographs by Mr. Galton, which may be pecualiarly useful in the study of anthropology. One of them is a typical family composite portrait composed of a mother and two daughters, in which all three faces are blended together. We are thus given a likeness of the female branch of the family; another, a blending of the father and mother, two sisters, and two brothers, gives the typical family group. Other pictures, in which the same principles are applied, give a typical group of engineer officers and a typical group of sappers.