The history of three-color photography (1925)

530 History of Three-Color Photography filter of certain density was required. The later plates show much less of this reddish tinge, and if the same filter be used, the action of the blue would be undercompensated, or in other words bluish pictures must be produced. This is a far more likely cause than the fading of the filter, which has a considerable stability to light. If the plate appears too dark generally and the colors dull, it may be due to either underexposure, which tends to give excess blue, or the plate was insufficiently developed at first, or the developer was too cold. Both these would naturally cause less primary reduction, therefore, there would be more than the correct quantity of silver halide reduced after reversal. Insufficient action would also tend to produce this, but this would be more apparent in the high lights. Overexposure produces pale colors with want of detail in the high lights, as there would be too much silver reduced at these points in the primary development, and thus too little after reversal. Too high a temperature of the developer, or too much ammonia, insufficient washing after the clearing bath will also cause the picture to appear too pale. If the picture shows no color, the plate was probably placed the wrong way round in the dark slide, that is with the sensitive surface facing the lens. If the colors appear correct after the second development and become paler on fixing, the duration of the second development was insufficient, or the plate had not been exposed long enough to white light. Out of focus images may be caused by incorrect position of the focusing screen, focusing without the compensating filter in position, and with some lenses to alteration of the diaphragm after focusing. If the silver intensifier is used too long, so that it becomes muddy, specks of metallic silver may be deposited on the film, or the whole film may appear yellowish. This may be cured by treatment with the neutral permanganate solution and fixing. A general rosy tinge is caused by too long washing after fixing, so that some of the green dye is dissolved. Very frequently deep shadows show a distinct blue tinge, and H. Lehmann105 ascribed this to the fact that with very low intensities of light, blue always develops before the other colors at the less refrangible end of the spectrum. Von Hiibl106 showed that this explanation was unsound, as a scale of greys was correctly reproduced by white illumination, both when exposed for a very long time to a poor light, and for a short time to a good light. He ascribed this fault to an incorrect compensating filter, and stated that many of those on the market were not of equal density. G. A. Le Roy107 suggested that when Autochromes were too bluish from underexposure, they should be immersed in a weak solution of the complementary color, such as Poirrier's orange II. Such baths must be used with care, and not allowed to act too long, or the remedy would be worse than the disease. Viewing and Exhibiting Screen-Plates. — The full beauty of a