This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Backing Sheets. A very handy method of backing plates quickly and economically is to use backing sheets. These are composed of stout material, such as tough brown paper, coated with a film of backing mixture. These sheets can be squeegeed or pressed into contact with the back of plates, removed before development, and used again until worn out. Backing sheets can be prepared as follows: Dissolve a quarter of an ounce of glycerine and half an ounce of gelatine in two ounces of hot water. Add enough India ink to make a deep black. Coat tough brown paper with this mixture, and squeegee on to glass to set. Cut this prepared paper to required size, and when wanted it can be rubbed with the palm of the hand into contact with the back of the plate. Remove before development.
177. Another efficient backing is provided by procuring a piece of black carbon tissue. Cut out a piece slightly smaller than the size of the plate to be used; moisten the tissue with water. Allow it to become as nearly dry as is consistent with its remaining limp, then again moisten the black surface, but this time with glycerine. As carefully as possible press the tissue against the back of the plate. It will adhere, and may be removed just before development.
178. Glycerine can be spread evenly on black paper (which comes in the box of plates), cut to the same size as the plate employed. These sheets can be pressed into contact with the back of the sensitive plate, care being taken to remove all air-bells. Remove this backing and rinse off all glycerine from the plate before developing. Be very careful not to get any glycerine on the film side of the plate.
Color Corrected Plates. Not until a thorough knowledge of the manipulation of ordinary plates has been obtained should the beginner, or even any photographic worker, attempt the use of plates which have been corrected for various color values. When thoroughly accustomed to the ordinary plate, of good rapidity, then the special plates, which will produce the proper color tones and tints, may be used. It is an added power and enables some work to be done which could not be made without it. Color corrected plates are otherwise known as ORTHOCHROMATIC and ISOCHROMATIC PLATES.
Advantages Of Color Corrected Plates. There is considerable advantage in employing color corrected plates in landscape photography. The ordinary plate, being much more sensitive to blue and violet than to green, yellow and orange rays, fails entirely to translate the colors of nature with accuracy into monochrome (one color). This failure would be more noticeable were it not for the fact that in addition to the colored light reflected from an object there is a proportion of white light. The defect is noticeable, however, even to an untrained eye, for it is a very common thing to see in the average landscape picture the unnatural darkness of grass, trees, etc.
181. Although it is not claimed that the orthochromatic plate with a ray filter gives a perfect rendering, there can be little doubt as to the improvement effected when the ray filter is employed with plates that are sensitive to yellow and green. The tone of grass and foliage is better, while the holding down of the blue by the ray filter enables white clouds and blue skies to be reproduced with very nearly their proper color value, and prevents the bluish haze of the distance from being exaggerated through over-exposure.