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.
Ray Filter. In photographing landscapes, the sky will be greatly over-exposed by the time a sufficient amount of detail has been secured in the foreground, as the plate is extremely sensitive to the blue rays of light. In order that a perfect register on the plate be made, and that the relative values exist between the sky and the foreground, some means must be employed to retard the action of the blue rays of light. This is accomplished by placing in front of the lens a yellow screen, or filter, which will absorb a certain percentage of the blue rays. The rays absorbed, or retarded from acting on the plate, depend entirely upon the density of the yellow filter. An extremely light colored filter may increase the exposure four times and such a filter is, therefore, called a four-times screen. A darker one may increase the exposure eight times, hence it is called an eight-times screen. Where the plate is corrected chiefly for yellow (as in the case with ordinary brands of orthochromatic and isochromatic plates) and where the view contains practically little blue - the predominating colors being green and yellow green - it will not be necessary to employ a ray filter. As a rule, however, the full value of color corrected plates is not realized until a ray filter is used in conjunction with them. It is not correct to state, as is sometimes done, that color corrected plates are of no advantage whatever unless a screen is employed, for there is always some advantage in using orthochromatic plates and often a very material improvement is obtained by using these plates, even without a screen.
183. In landscape work there is a tendency almost to "over-correction" if the yellow light filter is employed, unless the photographer is very expert in gauging the exposure.
184. For the average required ray filter a fixed-out and washed lantern plate, dyed in a solution of yellow color to a depth sufficient to increase the exposure from five to ten times, will give a screen suitable for most landscape work. (See Paragraphs 193, 194 and 195.)
185. THE BICHROMATE OF POTASH RAY FILTER is simple in construction, consisting of a neat metal ring, in which is mounted a brass cell, the top and bottom of which are made of two thin pieces of a high grade optical glass, with surfaces perfectly plane and parallel. The cell holds a solution of bichromate of potash. This construction is specially adapted to the three color process, inasmuch as the cells may be filled with fluids of any desired color, and the shades may be easily varied by strengthening or diluting the solutions, which do not degenerate.
186. This ray filter fits like a cap on the front of the lens, the most convenient position for attachment or removal, and is manufactured in four styles.
187. Style "A," shown in Illustration No. 27, is cork lined and fits over the hood of the lens. When ordering, send strip of paper just reaching around the hood.
191. In Illustration No. 28 we show the complete cell outfit, as manufactured by the Bausch & Lomb Optical Company.
192. This attachment, by absorbing certain rays from the light which would otherwise reach the lens, equalizes the effects which the various component colors in white light have upon the sensitive photographic plate. Therefore, practically speaking, the true color values of all objects are accurately reproduced in the monochrome picture. As is the case with all correctly made ray filters, over-exposure of the sky is prevented and detail in the clouds reproduced. Most of the rays of light which cause halation are absorbed, and thus halation is practically impossible. Distant objects appear distinctly in the image, even when photographed through miles of atmosphere.