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Behr, H. C., The Consolidated Gold Fields of South Africa, Limited, P.O. Pox 1167, Johannesburg, Transvaal, South Africa.

Bensusan, S. L., M.Inst.M.M., Equitable-building, Sydney, Australia.

Bhatt, Parvatiprasad Vishvanath, 55, Blenheimcrescent, Notting-hill, W.

Body, John Benjamin, M. Inst. C.E., Puente de Alvarado 15, Mexico City.

Bose, S., Deputy Superintendent, Central Jail, Jubbulpore, India.

Bostwick, H. R., Messrs. Collbran and Bostwick, Seoul, Korea.

Bott, John, 37, Herne-hill, S.E.

Bower, Edw. H. M., Port Office, Calingapatam, Ganjam District, India.

Brebner, Captain Charles William, Villa des Roses, Rose-hill, Mauritius.

Brelich, Henry, A.R.S.M., care of Messrs. Arnhold, Karberg and Co., Hankow, China.

Browne, Hon. John E. D., The Neale House, The Neale, co. Mayo, Ireland.

Brownell, Clarence Ludlow, 21, Hermitage-road, Richmond, Surrey.

Budge, Edward Barnard, B.Sc., M.Am. Soc.C.E., Engineer in Chief, 1st Section, Chili State Railways (F. C. del E.), Estacion Bella Vista, Valparaiso, Chili, South America.

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Deerhurst, Viscount, Dynes Hall, Halstead, Essex. De Marillac, Count Ernst, Wynberg, Cape Colony, South Africa.

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Etherington, John Francis, Hersham, Surrey.
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12,

Gilfillan, W. H., Surveyor-General's Department, Pretoria, Transvaal, South Africa.

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Leeds, Edward Lambert, The Brown Hoisting Machinery Co., 39, Victoria-street, S. W.

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Letcher, John Teague, Truro, Cornwall.

Letcher, Thomas Henry, St. Day, Scorrier, Cornwall. Lithgow, William T., Kingston Shipbuilding Yard Port Glasgow.

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The paper read was

SOME ASPECTS OF PHOTOGRAPHIC DEVELOPMENT.

BY ALFRED WATKINS.

Development is the process of reducing those particles of silver salt which have been affected by light, to the black or metallic condition.

A finished negative consists of a contrastor series of contrasts-between tones or densities. If the contrast obtained by reducing all the light-affected particles were always correct, the process of development would be simple and purely mechanical; for after devising a developer which did not attack the unaffected particles of silver salt, it would only be necessary to leave the exposed plate a sufficient time in the developer for it to do all its work, any longer time having no further action.

But practical photographers have long found out that with most plates and subjects a maximum amount of development, as above described, is injurious, and does not give the desired result, as the contrast between the tones is greater than represents the original object. A certain amount of judgment has therefore been necessary in deciding how far this reducing or developing-action should be allowed to proceed. I must point out that this judgment or control has been in the past -exercised in quite a different way from what I have indicated, and, as I shall try to point out, in a way the complications of which were quite unnecessary.

Instead of the photographer keeping the composition of his developer fixed, and con

trolling results by the time he allows it to act, he has been in the habit of keeping to his own time of development (varying, perhaps, only for temperature), and altering the composition. of the developer to get greater or less contrast, or, to put it in another way, to get greater detail in one case, or greater density in another case. The photographer also has got into the habit of considering that he possesses the power (by altering the composition of developer) of doing more work either on the upper tones (adding density), or, in another case, on the lower tones (bringing up detail), whereas, in nine cases out of ten, he has merely attained a result identical with what he would have got with an unaltered developer acting for a certain time. This older point of view is partly a legacy from wet collodion development, where the presence of free silver nitrate in the developer actually built up the high light deposits. in the negative, and partly arises from the use of an imperfect alkali (ammonia) in early dryplate development. For the volatile nature of ammonia often made it necessary to add more of the alkali during the progress of development, and its tendency to fogging often made the addition of a restraining bromide necessary. These two procedures (adding more alkali to stimulate development, and adding bromide to restrain fog), both due to an imperfect alkali, have been continued when the use of a fixed alkali makes either of them unnecessary, and mysterious advantages have been attributed to them. In short, the development procedure of a few years ago resembled the medical knowledge of the 18th century, being built up of fads and formulæ, with no guiding principles to direct it.

:

It was the memorable paper of Messrs. Hurter and Driffield (Journal Soc. Chem. Industry, May, 1890) which pulled the complacent practical photographer up with a jerk, and showed that his ideas on development would have to be re-constructed. The writers impressed on a strip of plate a series of exposures increasing in geometrical ratio, thus :1, 2, 4, 8, 16, &c., and investigated the effect of alterations in the developer. They found that the results of alterations consisted chiefly in variations of steepness of gradation (or contrast between the tones) attained, but that this was mainly a question of time, for all the developers (given sufficient bulk and activity) were capable of attaining the maximum steepness or amount of contrast if sufficient time to act were allowed. They named the degree of steepness or contrast the

development factor, but it was a weak point that the development factor was merely a record of a result attained, and not a help towards the attainment of the same result another time.

In the later paper by the same authors, published in the Photographic Journal, January, 1898, and following months, a most important light was thrown upon the action of a bromide as a restrainer, and the great differences between the results obtained by different experiments (especially as regards the speed of a plate) were traced to the peculiar action of a bromide in the developer.

I shall not attempt, in this paper, to follow up the many proofs furnished by Messrs. Hurter and Driffield, but shall outline my own interpretation of the simple principles of development, using some illustrations borrowed from my recently-published manual on the subject, and shall pass on to some points arising from my own trials and investigations.

SIMPLE PROGRESS OF DEVELOPMENT.

I throw on the screen a strip of plate which has received an increasing series of exposures in steps, from sec. to 512 secs. Incidentally this also shows the limits of the plate, for it will be seen that 64 is the maximum effective exposure, anything more than this having no increased action on the plate, while is the minimum effective exposure, anything less than this having no action at all on the plate. The whole problem of exposure-which I am not expounding in this paper-is to bring all the light impressions which form the picture. between these two limits. The upper strip is developed double the time, and the contrast between the tones is increased, for while the lowest tone has increased very little in opacity, the upper tones are greatly increased. Neither the maximum nor the minimum limits are altered by the longer development.

Fig. 1 is an imaginary section through an exposed film. Three exposures have been made on the plate, the part A being unexposed.

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velopment, those undotted being unaffected. I need scarcely explain that in a graphic diagram like this, no attempt is made at completeness; for instance, in the part D all the particles are shown light-affected, whereas with long exposures in a thick film only about 60 per cent. of the particles would be lightaffected.

In Fig. 2 an attempt is made to represent graphically what occurs when an efficient developer has one quarter accomplished its work, FIG. 2.

there being a comparatively feeble darkening in all three tones with little contrast between them. Fig. 3 indicates the stage when development is half over; and Fig. 4 when development is completed, and all light-affected

FIG. 3.

FIG. 4.

particles completely reduced to the black metallic state. With a well balanced developer there is no effect on those particles not affected by light, but an ill-balanced developer with excessive alkali will sometimes attack them, this being called fog. In these diagrams representing the simple course of development, it will be seen that the same proportion of work is done on each of the tones at each of the stages. But, as Messrs. Hurter and Driffield have pointed out, an arithmetical increase in the blackened silver results in a much greater (geometric) increase in its opacity or power of stopping light, and, therefore, in the natural course of development the contrast between the tones increases with the length of development. This rule applies with all developers, its limit being when fog commences and when all available light-affected silver is darkened in the darker

tones.

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CONTROL OF DEVELOPMENT BY TIME.

Although Messrs. Hurter and Driffield expounded the great importance of time in development, they gave no further help to uniformity in practice than the fact that, having once developed a plate to a required contrast or steepness of gradation, you could attain the same steepness (development factor) with another plate by using exactly the same developer, at exactly the same temperature, for exactly the same time. As it is difficult in practical work to ensure an exactly uniform developer (as regards alkali especially), and still more difficult to keep an exactly uniform temperature all the year, a practical working standard was still wanting. It is this aspect of development to which I have devoted most investigation.

In 1893, while testing a large number of plates for speed, and developing them together in one dish, I observed that some plates developed more readily than others, and attained contrast with greater rapidity; also that the image on these plates appeared more quickly than that on the others. In order to equalise the development, I adopted the plan of developing each plate for a fixed multiple of its time of appearance. In this particular case the time of appearance made correctly the allowance due probably to the different character of gelatines used in the emulsions. But this led

to a long series of experiments, in which I established the fact that the time of appearance made the correct allowance for alterations in the activity in the developer, due to the following causes :-Temperature (between 45° and 75°); alkali in developer; dilution of developer-pyro and amidol being an exception to this.

The time of appearance is the time elapsing between pouring on the developer and the first appearance of any trace of the image. The multiple used to attain the required result I have termed the multiplying factor. The multiplying factor varies with different developing agents, such as hydroquinone, pyro, metol, &c.

The multiplying factor is usually the same for different commercial plates, but I have fonnd lately some plates which require a higher factor to secure sufficient contrast. These plates are some which makers seem to turn out in their efforts to secure a high speed reading. and I notice that they are a deeper yellow than usual. It seems, therefore, that an increased proportion of an iodide in the emulsion alters the multiplying factor. I have found, by the way, that a little iodide of potassium in the developer very much alters the law of appearance, the image appearing almost as quickly at the back of the plate as at the front. bromide in the developer has quite an opposite effect. The use of a bromide or other restrainer in the developer alters the factor.

A

The experienced photographer using this timing system will sometimes alter the multiplying factor to bring an exceptionally wide range of tones in his subject within the limits of his printing process.

When investigating the laws of appearance, I found that the ratio of the appearances of all the various tones in an exposed plate was not altered by variations in the character and activity of the developer, and that if the times of appearance of a slip of plate exposed on the Hurter and Driffield plan, is plotted out in a similar to the Hurter and Driffield method way of plotting out densities, it is possible to read the speed of the plate by an observation of the diagram. I have made a recording instrument for observing these appearances, and recordiug

a diagram, but do not propose to follow up this branch of the subject in this paper.

To return to the practical question of controlling results by time of development, I throw on the screen prints from negatives of the same subject and exposure developed in a 2 grain pyro soda developer, grain bromide, for 34

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