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Screen-Plates — Historical and Theoretical Data 471
and blue are chosen, so that each line acts as an individual filter, and the same screen-plate serves as a reproduction filter. In 1, Fig. 119, the vermilion lines are covered and the spot appears blue-green, 2 appears purplered, 3 yellow, 4 blue and so on, 11 shows a blackish and 12 a whitish bluegreen, shades which are complementary to yellow-orange, but have a negative character.
If such a linear screen-plate is placed on a plate, equally sensitive to all colors, the rays reflected from a vermilion colored object will pass through a red line, but not through a green or a blue, so that only under this red line will the silver bromide be affected and blackened by subsequent development. If Fig. 119 represents a section through the two plates and "r," "gr" and "bl," the section through the red, green and blue lines, only the parts behind the red lines will be black after development and fixation, whilst those behind the green and blue will be clear glass. In a similar way with incident yellow-green and blue rays, only the silver bromide under the similarly colored lines will be affected and appear opaque in the negative. If yellow light, formed of a mixture of red and green rays, falls on the lines, the blackening will only take place under the red and green lines, for the blue absorbs the complementary rays before they can affect the silver salts. In 5 and 6 will be seen the action of the blue-green and reddish-violet rays reflected from the object.
White light penetrates equally all three lines, and causes an equal density, whilst under the black parts of the image the sensitive film will remain unchanged. The weakened white light from a grey object will as shown in 9, only produce partial blackening. If the color of the original does not correspond to any line of the screen and is not complementary to any, it will penetrate two colored fields with different intensity and cause different deposits. Yellowish-orange, for instance, as will be seen from 10, causes a deposit under the red and green lines. A whitish yelloworange, as shown by 11, acts through all three lines, whilst a saddened yellow-orange or brown, causes only slight deposits under two lines, as shown in 12. Thus the light incident on the plate is split up by the closely contiguous linear filters into three parts, and the deposit under the same represents the red, green and blue parts of the same. If a plate thus exposed is developed, fixed, and placed in contact with a similarly colored plate, so that the red, green and blue lines will fall in the same places, we shall see a negative or complementary colored picture. If, however, from this negative a transparency is made, and this covered with the linear color plate, a positive picture in the colors of the original must be formed, which can be easily understood by considering that Fig. 119 is looked through, and that the opaque parts are transparent and the transparent opaque. The opaque parts in the negative are now transparent, and therefore colored, and the quantity or the brilliancy of the colors is in ratio to the deposit on the negative.