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Subtractive Processes 629
standard tri-color red A, and 58. or B 2, we have 5800 to 6400 overlapping. This includes the golden yellow and 70 per cent of the orange. With Nos. 21, monobromofluorescein, and 45, or H, there is a small overlap from 5400 to 5500, which is practically in the middle of the greenishyellow.
The position of the overlap is important as regards the rendering of the yellows, which are next to white the most luminous colors, and the aim should be to include these as far as possible.
Assuming that the previous statement, as to the reproduction of the spectrum is correct, then the most obvious course is to use as printing colors those which shall be the sum of the absorption of each filter, that is to say, the red dye must match the colors absorbed by the green filter, and the green match the absorption of the red filter. One has to bear in mind, however, that the blues and deep violets are not recorded in the negatives at all, therefore, they can or must be ignored when dealing with the reproduction colors. The point is where should one locate the limits of color reproduction? On reference to the above table it will be seen that cyan blue ends at wave-length 4670, the limit of the indigo which is relatively a dark color. We may, therefore, assume that the whole of the spectrum from this point to the red end must be reproduced.
As we have assumed that the subtractive colors shall be anti-chromatic to the taking filters, we may limit the green to this particular point for the blue end, and set its limit at close to the middle of the orange, which is wave-length 6310. The sum of the spectral region between these two limits must be the color of the dye used for the green, or minus-red, record. Reference to Mees' "Atlas of Absorption Spectra" will show that acid green plus naphthol green is likely to be of value, for the former shows an absorption from about 6000, with a transmission band from 6500 to 7500, which can be cut out by the naphthol green. But at the blue end these two may transmit a little too far, so that the addition of a yellow, such as naphthol yellow S, may be required. It must be noted that we are limited to acid dyes, unless the silver image itself is to be dye-toned, when basic only must be used.
Treating the red end in the same way, we find that the limit of the yellow is 5270, but this might make the red too much of an orange, that is to say any deep reds in the picture would be rendered as scarlets, so that if we shift this point further towards the red, to about 5500, we shall find that the scarlet BB extra N or B, plus a yellow dye to cut out the blue transmission of both these dyes ought to be satisfactory. Or one might use rose Bengal, plus a yellow, for this has a very clean, sharp cut just about the point we want.
For those who like to have some standard to work to, other than the spectrum, it will probably be found that for the green dye, a mixture that will match Wratten & Wainwright's Xo. 52 filter, will be a good guide.