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Transparencies in Various Tones 574

development, colour, image, colours, time, silver and red

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TRANSPARENCIES IN VARIOUS TONES 574. Warm Tones by Development. Images in warm tones obtained by direct development are formed of colloidal silver of ultra-microscopic size and bright colour, yellow, red, or purple, mixed with a greater or lesser proportion of black reduced silver of microscopic size. These images of very fine grain are obtained by devel opment in a solution containing at least a small quantity of a solvent of the salts of silver- sodium sulphite, ammonium bromide, or am monium carbonate either added to the developer or formed in the bath by reaction between the ammonium bromide and another carbonate. This solvent action is facilitated in proportion to the fineness of the grain of the emulsion itself (non-ripened emulsions or only slightly ripened, prepared with silver chloride or silver bromide, or a mixture of these two halides), the size of the grain exercising a much greater influence than the nature of the halide. 2 It will be seen later (§ 578) that positive emulsions intended for black tones can be used for the production of images of warm tones by development in solutions to which have been added more power ful solvents, or in more concentrated solutions. But the colours so obtained are not usually so pure.

In a developer suitable for obtaining warmn tones, the image changes in colour during the various phases of development on account of the progressive increase in the size of the par ticles of reduced silver. These colours, examined in white light after fixing, washing, and drying, are successively- Yellow; Red ; Brown ; Sepia ; Black.

This succession of colours is absolutely inde pendent of the time of exposure and of the light; the same colours are found, time after time, in any one developer at the same temperature with identical times of development (A. Godcrus, 1896). In developing solutions which differ slightly in composition—such differences as are due to variations in strength of commercial materials, or in solutions prepared by varied dilution of stock solutions—the same colours occur when the contrast of the image reaches the same value, or the development factor is the same (note to § 202) (S. H. Wratten, 1910). It is only for convenience in working—i.e. for passing more or less rapidly from one colour to another—that it is recommended to use devel opers differently constituted, or diluted in different proportions, for obtaining the various colours mentioned above.

However, at least with certain emulsions, a very dilute developer does not give tones be yond red or brown, however long development may be prolonged, in spite of the increase in gamma which results from this prolongation of development (L. Lobel and M. Dubois, 1929).

The fact that the colour depends essentially on the contrast factor of the developed image enables one to judge in advance that negatives of different degrees of contrast are not equally well suited for obtaining images in various colours. A negative which will give a good transparency in sepia, the colour most frequently desired, with good tone value and contrast, will yield only a weak red image. And, inversely, a negative producing a good red positive with correct contrast will give, necessarily, a sepia image with far too strong contrasts. On the other hand, the gradation not being the same in all brands of plates, a certain negative will be better suited to one than to another for obtaining a particular colour, whilst another negative of different character might very well produce a more satisfactory result on the brand of plate which had been rejected in the preceding example.' These differences become speedily evident after a certain amount of experience has been obtained.

575. Exposure. The statement that the colour of the image is independent of the under given conditions of illumination, does not imply that the exposure should be the same whatever may be the colour desired.

Under the conditions of development deter mined upon, the density of a certain part of the image is greater in proportion as the ex posure time has been greater, or, more exactly, the light-action, that is, the time of exposure multiplied by the intensity of the light. For a given degree of development, the image may not appear in those parts of the subject which form the densest parts of the negative unless the exposure has been greater than a certain mini mum—a minimum which varies with the time of the development on account of the regression of the inertia (§ 337) in a developer rich in bromide, as is usual in developers for warm tones.

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