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464 History of Three-Color Photography
of a screen-plate, Mees and Pledge22 stated that the following must be taken into account, and that each must be considered separately, and the necessary conditions fulfilled. All can be tested without taking pictures at all, and if they are not fulfilled the color will be wrong:
1. The size of the units. For regular screens, these should not be larger than 1/300 of an inch, nor smaller than 1/600. For irregular screens not larger than 1/900, nor smaller than 1/2000 of an inch. It is quite needless to strive for exceedingly small units.
2. The interstices. If these exist at all, they must be filled in ; white interstices are fatal, even if they only occupy one-twentieth of the area of the screen-plate.
3. The colors of the units. These must be primary red, green and blue-violet, and conform to the conditions explained.
4. The relative area occupied by each color. This must be adjusted to fulfill the first black condition.
5. The emulsion. This must be coated, for which purpose the varnishes will have to be selected, as they must not act upon it. Turpentin and ether, especially the former, are inadmissible as solvents; resin varnishes are suspect.
6. The sensitizing. This must be performed so that the actions under the red and green filters are equal.
7. The compensator. This must finally be adjusted to fulfill the second black condition.
The following notes apply to Autochrome plates, for these alone were for some time the only plates available commercially.
With reference to the fineness of the screen elements, it may be of interest to summarize, as far as possible, available data, and Mees and Pledge gave some interesting figures. The Joly line plate was ruled with lines 0.12 mm. in width, while the McDonough plate lines were 0.08 mm. These plates are no longer available. The Autochrome plate is covered with starch grains, and numerous measurements have been made of these. The author23 found that they varied from 0.006 to 0.025 mm., which gives as a mean 0.015 mm. and this was confirmed by Mees and Pledge.24 L. Benoist25 found that the finest grains were 0.008, the medium 0.0105 and the coarsest 0.0132, which gives as a mean 0.0106 mm.
With regards to the distribution of the grains, theoretically this should be uniform, as the stained starch grains are mixed to satisfy the first black condition as far as possible, but as pointed out by Mees and Pledge, according to the law of probability, there will always be clumping of the grains of one or all colors. If, for instance, counters of three different colors are placed in a bag and two are withdrawn, then the chances that those two counters will be of the same color are 1 in 3 ; the chances that three counters will be the same color are 1 in 9; four, 1 in 27; the chances that twelve will be the same color being about 1 in 120,000. In