Photographic optics and colour photography : including the camera, kinematograph, optical lantern, and the theory and practice of image formation / by George Lindsay Johnson.

George Lindsay Johnson

Date:: 1909

Credit: Photographic optics and colour photography : including the camera, kinematograph, optical lantern, and the theory and practice of image formation / by George Lindsay Johnson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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$Eoughly speaking, for soft contrasts use three-fourths of the normal factor; for strong contrasts, add one-fifth to the normal factor. Example.—Metol-quinol is used as the developer. The image first appears after 20 sec. Since the factor is 12, the plate must be left in the developer for 20 x 12 sec, is. 4 min. If soft contrast be desired, the plate must be left in for 20 x 9 sec, and for hard contrasts, for 20 x 15 sec. Rule for factor developing.—Multiply the number of seconds that have elapsed between pouring on the developer and the first appearance of the image by the factor number. The product gives the time that the plate should remain in the developer. Double emulsions, such as Cristoid films and Thomas's plates, require at least double the time of the factor. Other plates, whether slow, fast, or isochromatic, do not appear to affect the result. Rule for combination developers.—If equal quantities of each be used, half the sum of the two factors will be the factor of the mixture. If the mixture contains unequal parts, proceed as follows:— Let / = factor number of solution A ; /' = factor number of solution B ; x = number of ounces of A; y = number of ounces of B. Then the combined factor number p +fy x + y [87] Example.—A mixture is made of 4 oz. of hydroquinone and 1\ oz. of metol. What is the combined factor F ? The factor of hydroquinone is 5, that of metol is 30, therefore -pi fa +/V 4x5 + (1,5 x 30) This does not hold strictly true with pyro developers, which affect the speed of other developers in a different way. As regards the ultimate image, all developers appear to give the same, or nearly the same, result, but the rate at which$