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Screen-Plates — Historical and Theoretical Data 463
trials that the distance of blending (Verschmelzungspunkt) was not the same for all three colors; the blue and green tended, in the majority of cases, to blend before the red ; in two cases the red was visible long after the others had blended into a uniform tint. This difficulty of blending of the reds may account in part for the apparent predominance of the red units in some screen-plates. In connection with this subject it should not be overlooked that the eye is not achromatic, and one must also take into account the Purkinje phenomenon.
Mees and Pledge19 pointed out that taking 20 cms. as the viewing distance, the screen period will be 0.2 mm., that is the separate units of the screen will be invisible if they are 0.66 mm. in diameter. This is assuming that regular screens are used, but with irregular units one has to take into consideration the clumping of grains of the same color. This effect may be due to improper mixing, but it is inevitable by the law of probability.
The thickness of the screen element necessary to obtain the requisite depth of color also settles the diameter, in consequence of parallax. If the thickness is equal to the diameter of the element, or to the width of one line, then any light ray entering the screen-plate at an angle above 10 degrees will be subject to considerable parallax error, as it will have passed not only through the element through which it ought to pass, but also diagonally through a portion of the adjacent unit before passing into the proper screen element ; so that a green ray, for instance, would pass through the top surface of a red unit, and suffer considerable absorption before passing through the green element.
With gelatin as a medium of the color this is of great importance, as it is difficult to obtain a sufficiently deep screen with less gelatin than 1 ccm. of a 5 per cent solution per 20 qcm. ; and the thickness of this coating, whilst wet, will be 0.5 mm., but when dry only 0.025 mm., which gives 40 lines of square section to 1 mm., or the limiting fineness of the lines is 1000 to the inch.20 If irradiation occurs in the emulsion the effect will be to reproduce all over-exposed sections of the plate as white, and an increase of exposure of 4 times is sufficient to dilute a color with an equal amount of white, an effect which would require at least 40 times the normal to produce by penetration of the screen elements.
As regards the emulsion, this must be of reasonably fine grain; but it need not necessarily be slow. It must be sensitive to the whole of the spectrum, and the sensitiveness must be so adjusted that the deposit beneath the red and green filters is equal in intensity. But this, and the effect of the blue, can be adjusted by the compensating filter, so that if greys are photographed, greys will be reproduced on the plate, and this is an excellent method of testing as to the compensation correctness. This, Mees signalized21 as "the second black condition."
Summing up the necessary adjustment of factors in the construction