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Sub tractive Processes 627
Wainright. We can thus confine our attention to subtractive work alone. For negative work the standard filters, recommended by the above firm, are their Xos. 22 or E 2, and 60, also known as P, and the transmissions of these is shown in Fig. 175. If these be placed on one chart, we shall see how much of the spectrum is passed by them and the amount of the overlap.
As a guide to the colors we may take Helmholtz's table of the distribution of the colors, as follows :
Red, A-C wave-lengths 7600-6560
Orange C-C^D wave-lengths 6560-6060
Golden-vellow, C^D-D1/6E wave-lengths 6060-5790
Yellow, D1/6E-D1/3E wave-lengths 5790-5680
Greenish-vellow, D1/3E-E wave-lengths 5680-5270
Green, E-'b wave-lengths 5270-5180
Blue-green, b-F wave-lengths 5180-4860
Cyan blue, F-F1/3G wave-lengths 4860-4670
Indigo, F1/3G-G wave-lengths 4670-4300
Violet, G-H wave-lengths 4300-3970
From Fig. 176 it will be seen that the actual overlap for the above filters is from 6180 to 5580. so that we have some of the orange, the whole of the golden yellow and yellow, and about one-fourth of the greenishyellow. Now it is important to bear in mind that it is the shadows of the negative that print, therefore all the blacks in the diagrams used, represent those parts of the positives in which colors will appear, and as it is usual to employ red and green as the printing colors, we must think of these blacks in these two terms. If we combine the above figures into one diagram. Fig. 176, and conventionally represent red and green, the subject may be a little clearer, and it will be seen that in the overlap we must have dyes that will in thin films pass these colors.
If we assume that we have to reproduce the spectrum as given by these filters, then obviously the printing colors should be antichromatic to the taking filters, and this applies to all two-color filters from the theoretical standpoint. If this applies to the spectrum, then it should also apply to natural objects. It may be thought at first sight that we cannot under the above circumstances obtain a pure yellow, whereas if the above two filters be superimposed, it will be seen at once that the transmitted color is yellow, though saddened by black, that is, of lowered luminosity. This statement applying to the light seen through the filters themselves superimposed ; and the lighter the hue of each filter is made the brighter the yellow becomes, that is to say, the less black there is.
If we take two other filters, that were suggested for two-color work42 Xos. 29, or F, and 44, or minus red 4, we shall find, that there is no overlap, in fact, from 6100 to 5850 is not recorded at all. This region is the golden yellow, and includes the D lines, and superimposing these filters in the same way, yellow is not formed. In another pair, Nos. 25, or the