The limitation of the size of the filter element is determined by the invisibility of the element, which has been previously dealt with, and this should be not larger than in., or in the case of irregular grain screens about one-tenth of that, because of the dumping. One of the factors governing the minimum size of grain is the thickness of unit necessary to give sufficient depth of colour, which is determined as to its diameter by parallax. If the thickness is equal to the diameter, any ray passing through the screen at a greater angle than roc' will not only pass through the particular element which it first struck, but also the next, as shown in the following diagrams.
A shows a screen plate in which the thickness of the filter elements 12, B (red, green, and blue) is equal to their diameter. The ray striking the blue element B passes only through this ; but as the emulsion is of slightly greater refractive index it encroaches slightly on the neighbouring green element, and therefore in the final result the blue would have a very slight green tinge. In diagram B the thickness of the elements is one and a half times their diameter, and the ray here passes not only through the blue, but through nearly the whole of the green ; therefore the final colour would be strong greenish blue. This is an important point because when using a bichromated colloid, such as gelatin; it is difficult to get sufficient depth of colour with less than 54'5 mins. of 5 per cent. gelatine solution to every io sq. in. (= r cc. per zo qcm.), which gives a thickness of dry film of in. The second factor limiting the size of the elements is the irradiation of light in the emulsion film itself, by which the light may be scattered from the silver halide particles, underlying the par ticular screen element, to those under the adja cent ones.
Finally, in order to obtain correct colour rendering, assuming that the first black condi tion is fulfilled, it is essential that the action of light should be to give equal deposit under all three elements. Now, as it is not possible to
sensitise any emulsion for red and green, so as to give equal sensitiveness to these regions as to the original blue-violet sensitiveness of the emulsion, therefore a compensating filter is used to cut down the excess blue-violet sensitive ness, and at the same time even up the sensitive ness of the red and green. This is called the " second black condition." The final speed of a plate is determined by (a) the speed of the emulsion, (b) the multiplying factor of the screen, and (c) the multiplying factor of the compensator. Mees and Pledge give the following table of these factors :— They also give the following very valuable summary of the essentials to be fulfilled in the manufacture of a screen plate : (I) The size of the units.—For regular screens these should not be larger than in., nor smaller than in. For irregular screens not larger than .6„, in., nor smaller than in. 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 arc of the screen plate. (3) The colours of the units.—These must be primary red, green, and blue violet. (4) The relative area occupied by each colour.— This must be adjusted to fulfil the first black condition. (5) Emulsion.—This must be coated, for which purpose insulating varnishes will have to be selected, as they must not act upon it. Turpentine and ether, especially the former, are inadmissible as solvents ; resin varnishes are suspect. (6) The sensitising.—This must be performed so that the actions under the red and green filters are equal. (7) The compensator.— This must be adjusted to fulfil the second black condition.