Instruction in photography

Front matter

THE

HOMON-PlCKARD

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Preface To Ninth Edition

A few remarks are necessary, by way of explanation of some of the changes that have been made in this edition. The gelatine process having so firmly got possession of the photographic public, it has been awarded the place of honour in the description of negative-making processes, relegating the description of the wet process to a more subsidiary position. Certain omissions in some almost obsolete processes have become necessary to keep the pages within reasonable limits, and to make room for various new chapters. The French system of weights and measures which was given in the last edition has been discarded for measures by parts. It has been considered that the grain is the most convenient standard to adopt, and, for simplicity's sake, the weight of 1 ounce of distilled water has been taken as 440 grains instead of 437J grains, a difference which is not practically felt when dealing with photographic processes. It is much to be desired that dealers should make measures graduated into parts. The present 20 ounce measure would have a capacity of 8800 grains by weight very nearly, and by increasing it slightly it could be graduated into 10,000 parts. The ordinary 4 ounce measure could also be readily slightly increased in size to show 2000 parts. Such graduated measures would at once enable photographers to make up solutions according to foreign formulae, the gramme being taken as a grain, and the cubic centimetre as one part.

Photography for scientific purposes has passed the experimental and what may be called the qualitative stage of its existence, and in this edition will be found a description of making measurements which convert it into a quantitative agent.

William de W. Abney.

PKEFACE TO TENTH EDITION

The publishers' call for a new edition has enabled me to bring the work, it is hoped, quite up to date. This has involved the addition of a large number of pages, which include such subjects as ' Photo-block work,' ' Three-colour printing,' etc. Some additions have also been made to the theoretical side of photography.

The subject of lenses has been treated more fully than before, and substantial extracts have been made from Mr. Child Bayley's history of their evolution, which appeared in the catalogue of the Crystal Palace Photographic Exhibition. To him I owe my thanks, and also especially to Mr. J. Mcintosh, who has rendered me most valuable aid in seeing the work through the press.

William de W. Abney.

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Contents

CHAP. PAGE

I. ACTION OF LIGHT ON SILVER COMPOUNDS . . 1

II. THE ACTION OF THE SPECTRUM ... 5

Chapter I Action Of Light On Silver Compounds

Nearly every metallic or organic compound undergoes some change in the presence of ordinary light. The change may be visible to the eye, as in the case of the darkening of silver chloride or the bleaching of dyed stuffs ; or it may be invisible, and only to be ascertained by the behaviour of the compound when certain chemical agents are brought in contact with it, as an example of which we may take the case of silver chloride when the exposure is not sufficiently prolonged to have caused a visible darkening. The evidence that a change has been produced in the latter is as strong under these last circumstances as in the former, and, by logical deduction, we arrive at the fact that any difference between a visible change and one which requires the application of a developing agent is only in the number of molecules, composing the compound, which are altered.

It will be evident that when an image of, say, some white body in which there are differences in light and shade is formed by a lens on a surface of a compound which can be affected by light, the amount of alteration produced will depend on the different gradations of light acting on it. Similarly, if the light be filtered through a screen in contact with such a surface, and which cuts off different quantities of light, the amount of alteration at the different parts will depend on the quantity of light reaching each of them.

The image of which cognisance can only be taken by the

B

LATENT OR PHOTOGRAPHIC IMAGE application of a developing solution is often inaccurately called the " latent " image ; it is far preferable to call it the " photographic " image. These terms are more particularly applied to the invisible change which takes place in a film containing or formed by a compound of silver ; and such compounds as are capable of being impressed by an image are termed " sensitive. "

The sensitive salts of silver which are usually employed in photography are, the iodide, the bromide, and the chloride of silver ; and they are the compounds which, up to the present, are found to possess the most advantages. There are others which are rarely used, and to which we may refer further on.

Silver iodide (Agl) can be formed in two or more ways — by the action of a soluble iodide on a soluble salt of silver, or by iodine vapour upon metallic silver. The last method is that which was employed in the old Daguerreotype process \ the first is that which is used in ordinary photography. In the last case —

Silver and Iodine form Silver Iodide 2Ag + 21 = Ag2I2

In the former, the soluble iodide of a metal, such as cadmium, or of an alkali, such as ammonium, etc., is brought in contact with a solution of silver nitrate. The iodine, having a strong aflinity for the silver, forms silver iodide, setting free the nitric anhydride, which, in its turn, combines with the metal originally in combination with the iodine. Chemically, it is expressed thus —

Cadmium Iodide and Silver Nitrate form Silver Iodide and Cadmium Nitrate Cdl2 + 2AgN03 = 2AgI + Cd(N03)2

If we were to substitute bromine (Br) or chlorine (CI) for the iodine (I), and a bromide or chloride for the iodide, in the above descriptions, we should describe the formation of silver bromide and silver chloride, the decomposition taking place being similar.

The chemical change that takes place in these three haloid salts, as they are called, by the action of light, whether visible when long, or invisible when short exposure is given to it, we have very good reason to believe to be the formation of a silver sub-haloid * that is, a compound in which one atom of the halogen iodine, chlorine, or bromine is separated from the original molecule, in which two atoms of these are supposed to be combined with two atoms of silver. Amongst other proofs

SUB-HALOIDS— HALOGEN ABSORBENTS which might be brought forward as to this fact we have, in the case of silver bromide, the fact that when it is given a prolonged exposure to light, bromine is smelt, and chemical tests confirm its presence. Thus —

Silver Bromide gives Silver Sub-bromide and Bromine 2AgBr = Ag2Br + Br

We may substitute the chloride or the iodide of silver in the equation for the bromide, and we shall have sub-chloride or sub-iodide formed, with the liberation of chlorine or iodine respectively, though in the case of the iodide the reaction may not, perhaps, be quite so simple.

The fact that there are certain reducing substances which will reduce the salts of silver to the sub-salts, and that such a sub-salt behaves under the action of a developer in the same way as the compound formed by the action of light, is further evidence, if more be required, that, in the latter case, the sub-salt is also formed.

It must not be supposed that this chemical change takes place in the whole of the silver haloid present ; far from it. The change, in the case of a short exposure, must take place in an infinitely small and unweighable part of it.

The reduction of the haloid, such as silver bromide, to the sub-haloid takes place with much greater facility if some body be present with the sensitive salt which will at once take up the atoms of the halogen liberated, and, in arriving at the greatest sensitiveness of the silver salt, it is always endeavoured to secure the presence of a body which has the greatest avidity for the halogen, barring only those which would, without the action of light, take it from the silver salt. In the case of the iodide, the halogen absorbent is almost an actual necessity. If, for example, a film of pure iodide of silver be prepared, as in the wet collodion process, and after immersion in the silver bath be washed and treated with iodide of potassium, or iodine in water, and again washed to free it from all excess of soluble iodide or iodine, such a change as that indicated above will take place with extreme difficulty, more especially if dry ; but if such a plate be treated with an organic substance, such as beer, or an inorganic substance, such as potassium nitrite, the silver iodide is able to part with its atom of iodine as indicated. In the same way, if silver nitrate be present, it acts as an iodine absorbent. In the case of the organic matter, we have a com-

4 NECESSITY OF HALOGEN ABSORBENTS OR SENSITISERS pound formed with the iodine. One atom of iodine eliminates one of hydrogen, or an hydroxyl (HO) group from the compound, and takes its place, and another combines with the hydrogen or hydroxyl liberated to form hydriodic acid (HI), or this with nascent oxygen. With potassium nitrite, we have —

Iodine, Potassium Nitrite, and Water* form Hydriodic Acid and Potassium Nitrate 21 + KN02 + H20 = 2 HI + KN03

When silver nitrate is the absorbent, as in the wet collodion process, the reaction is somewhat diff erent. It is usually considered to be as follows : — Multiplying the iodine by 6, we have six atoms of iodine (61) coming in contact with six atoms of silver nitrate (6AgNo3) and water, and then —

Iodine, Silver Nitrate, and Water produce Silver Iodide, Silver Iodate, and Nitric Acid t6I + 6AgN03 + 3H20 = 5AgI + AgI03 + 6HN03

To Dr. Vogel | must be given the credit of laying down the above law regarding the necessity of absorbents of the halogens (I, Br, and CI), and he has rightly named such bodies sensitisers. With silver iodide, under ordinary atmospheric conditions, as we said, such an absorbent is absolutely necessary ; but with silver chloride or bromide, the chlorine or bromine will be evolved without it, probably on account of the moisture, which is always ready to combine with them on liberation.

* Plates prepared with potassium nitrite always contain a certain amount of moisture, owing to the hygroscopic nature of the salt.

t When bromine and chlorine are liberated from silver bromide and chloride respectively in the presence of silver nitrate, the reaction that takes place may be somewhat different, but not essentially so. In the above equation it is possible that oxygen is liberated, and no iodate formed.

+ Photographic News, 1865, page 209.

Chapter II The Action Of The Spectrum

We now come to a part of our subject which the beginner may not at first understand and appreciate, though it is in reality most important in its bearings on photographic processes.

When a thin slice of light traverses a prism, it is separated into all its coloured components, and forms what is called a spectrum. The colours pass imperceptibly from one into the other, but, for the sake of perspicuity, they have been divided into seven principal colours. These are red, orange, yellow, green, blue, indigo, and violet. The spectrum is not confined to the rays that are seen, but extends beyond the red. At the violet end also the visible rays do not give the limit of the spectrum. If a card be washed over with a common lubricating oil, such as is used for bicycles, and placed in the spectrum, rays beyond the violet which are usually invisible will flash out, becoming visible. A solution of sulphate of quinine in water acidulated with tartaric or sulphuric acids will also cause the same effect. The spectrum has its greatest brightness or luminosity in the yellow, and fades off towards the red and the violet. Fig. 1 shows approximately what are the ratios of brightness at various parts of the spectrum of direct sunlight at mid-day in summer as determined by the author.* The brightest parts of the spectrum are not those which act most strongly on the salts of silver ; indeed, the yellow scarcely affects them ; it is the violet and blue, which, as will be seen from the figure, have very small luminosity, that affect them most. Experiment has shown that only the action of those rays which are included between the green and the ultra-violet need be considered as

* Bakeriaii Lecture, 1886. 5

ACTION OF THE SPECTRUM ON SILVER SALTS 7 making any considerable change in the silver compounds which are ordinarily employed by photographers. Those rays of light which do effect a change (visible or invisible) are often termed " actinic " or " chemical rays " ; all others, non-actinic. These terms are, however, misleading, except when properly guarded by mental reservations. It entirely depends on the sensitive compound employed as to which rays are " actinic." It will be seen, for instance, that to one modification of silver bromide * all the rays of the visible spectrum, and also part of the invisible infra-red, and ultra-violet, are " actinic," though, for the ordinary silver bromide, the red and infra-red may be classed as " non-actinic."

Most silver compounds — indeed, almost all sensitive compounds— are affected by the ultra-violet rays, and, in some cases, the change produced is more marked at this part of the spectrum than at others. The rays beyond the violet and below the red are usually called ultra-violet and infra-red respectively.

In order to determine as to what rays any particular compound is sensitive, appeal must be made to the spectroscope as applied to photographic purposes. When only approximate knowledge is required, the sensitiveness of a compound may be estimated by the darkness or the opacity of the print or negative obtained by an exposure to the spectrum ; but when a really accurate determination is required, resort must be had to some method of measurement. In fig. 2 we have an eye estimation of the amount of blackening or density of deposit on various compounds caused by the different spectral rays. The blackening or density is shown by the height of the curve above the base line.f It will be noticed that the effect of the direct action of light as shown by printing is given in Nos. 1, 3, 5, 7, and 10. In these cases the sensitive salt was formed in ordinary paper by salting the paper, as will be described in a subsequent chapter, and then floating it on a solution of silver nitrate, and drying. We may here state that if a collodion film had been used instead of paper, the effects would have been precisely the same as shown in Nos. 2, 4, 6, 9, and 11.

* Phil. Trans., 1881, Bakerian Lecture ; also Photographic Journal, 1881, page 95.

f Proc. Royal Society, vol. xxxii. ; Photographic Journal, May 1882.

ACTION OF THE SPECTRUM ON SILVER SALTS

AgI + AgN03 Print.

Developed.

AgBr + AgN03 Print.

AgBr Developed (long exposure)

AgCl + AgNO, Print.

AgCl Developed.

Agl + AgBr + AgN03 Print.

3 Agl + AgBr

Developed.

Agl + AgBr

Developed.

3AgI + AgCl +

Agl + AgCl Developed.

Fig. 2.

ACTION OF THE SPECTRUM ON SILVER SALTS 9

No. 2 shows the chemical effect of the spectrum on pure silver iodide, developed by any method.

No. 4 shows the spectrum impressed on silver bromide, developed by any method.

No. 6 shows the spectrum impressed on silver chloride, developed by any method.

No. 8 shows a silver bromo-iodide wet plate.

No. 9 shows a silver bromo-iodide dry plate, developed by any method.

No. 1 1 shows a silver chloro-iodide dry plate, developed by any method.

No. 1 shows a print on silver iodide paper in the presence of free silver nitrate.

No. 3 shows a print on silver bromide paper in the presence of free silver nitrate.

No. 5 shows a print on silver chloride paper in the presence of free silver nitrate.

No. 7 shows a print on silver bromo-iodide paper in the presence of free silver nitrate.

No. 10 shows a print on silver chloro-iodide paper in the presence of free silver nitrate.

The reader should note the limited range of the silver iodide,, and even more so of the silver chloride. There is also a remarkable fact in connection with silver bromo-iodide and silver chloro-iodide. When exposed wet, these salts combine the sensitiveness of the two salts ; but when exposed dry, the bromine or chlorine liberated by the action of light destroys the sensitiveness in that region of the spectrum (near G) in which the effect on the iodide is most marked. This is probably caused by the bromine or chlorine attacking the sub-iodide. Thus —

Silver Sub-iodide and Chlorine or Bromine {,'h'es Silver Iodo-chloride or Bromide Ag2I + CI or lit Ag2ICl or AgJBr

This has an important bearing on the dry plate process, in which such mixtures are to be found, and will be referred to in another chapter. The writer has made quantitative determination of the sensitiveness of different compounds to the spectrum, the method of obtaining which will be better understood when the subject of photometry is considered.

A fact to remember is that the action of light on a sensitive

10 CONSERVATION OF ENERGY — ORTHOCHROMATIC DYES

salt of silver is increased by heating the silver compound * during exposure. The effect of the heat is to cause the halogen to be more readily liberated than when the haloid salts of silver are cool, and the effect of cold is to retard the liberation of the halogen rendering the salt less sensitive. (See Camera Club Journal, 1895.)

We must here say a word regarding the ordinary action of light on silver salts which are dyed with certain dyes, on which orthochromatic photography, as it is called, depends. The law which is known as the conservation of energy requires that chemical action in a substance can only be effected by those rays of the spectrum which it absorbs. Thus, on page 8, the curves given of the darkening of the different salts of silver in various parts of the spectrum are a rough indication of the amount and kind of coloured rays which they absorb, more or less completely. Now, not only may these haloid (bromide, iodide, and chloride) salts of silver, but many other organic silver compounds, be changed by the action of light. There are certain dyes which can combine with silver nitrate to form organic salts of silver, a typical one being eosine, which is an organic bromide salt. When viewed in the spectrum, it is found that this salt absorbs the green rays. On exposing a plate coated with a mixture of this organic salt of silver with one of the haloid salts to the spectrum, it is found that, on development, a deposit of silver is precipitated exactly on that part where the green rays fell, together with a deposit on those parts where the part of the spectrum fell to which the haloid salt is sensitive. Thus, suppose the compound was a mixture of silver chloride and " eoside of silver," we should have, besides the curve 6, fig. 2, a small mountain lying between F and D. If silver iodide were mixed with the eoside of silver, we should have the same mountain in the green, together with that shown in curve 2, fig. 2. That this is the case may be seen by fig. 3.

It is not only silver nitrate, however, which will combine with the dyes. By use of ammonia, which acts as a partial solvent of bromide and chloride of silver, a compound of the bromide or chloride of silver with the dye can be formed, which may be a mechanical compound or lake. The part of the spectrum absorbed by this compound when it exists is not * Photographic Neivs, May 1884.