TESTING A DEVELOPER 393. Method for the Comparative Testing of Two Developers.' The procedure generally adopted by photographers to compare two developers consists in exposing two identical plates or films on the same subject with the same time of exposure ; one of these is developed in each developer and the images so obtained are then compared. This method is not very satis factory because it is not possible to place side by side the parts of the two negatives which have received the same exposure ; comparison is therefore difficult. The test is much more useful if it is carried out on a scale of uniform tones obtained by exposing a plate or film in successive bands to intensities which correspond respectively to the shadows, half-tones and high-lights of average subjects ; after cutting this negative into several identical strips, each comprising a complete scale of tones, strips are developed for different times in each of the two developers. After fixing and washing, corre sponding tones are placed side by side for comparison.
Take a plate or film (preferably 7 x 5 in.) or a piece of sensitized paper, place in a printing frame at such a distance (determined by a preliminary trial) from a weak source of light that after two seconds' exposure a normally developed emulsion just shows a trace of image when compared with a portion which has not been exposed at all. An opaque card held in front of the frame allows different times of exposure to be given to the various parts of the plate. At the start the card covers a band about half an inch wide and parallel to the longer side. This band will eventually be used for comparing the amount of chemical fog resulting from the various conditions of development. After two
seconds' exposure, the card is pushed on about half an inch, and this is repeated after periods of 4, 8, 16, and 32 seconds, counting from the start of the first exposure. 1 When this has been done, cut the plate or film into a certain number of pieces. Fig. 171 shows such pieces after different times of development in a mt'tol developer E and in a hydroquinone developer 11, and exhibits the characters of the two developers. The image appears very quickly in the metol developer, whilst in the hydroquinone developer it appears very slowly, but the density then increases more quickly, so that the two negatives are almost identical after a sufficiently long time of development.
394. Applications of the Practical Test. The density of the area A, which has been protected from all light-action, gives the amount of fog produced by development. The density at B, slightly exposed, corresponds with the shadows of a subject and records in some measure the power of the developer to render details in the shadows, whilst the difference in density between B and F shows its power to yield con trast.
When a comparison is made, at the same temperature, of two developers prepared by the same formula but with two different samples of the same developing agent, the times of appearance of the image are inversely propor tional to the respective contents of the active substance in the samples. This furnishes a simple test of the purity of such substances.
The same methods of working will serve to