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22 History of Three-Color Photography
importance in considering the adjustment of light filters, does not influence the character of the reproduction or minus colors.
There is one method of dealing with the reproduction colors or printing inks, which possibly possesses some little merit of novelty, which so far as it has been possible to trace has not been dealt with in this particular form. If we take the spectrum as the subject to be reproduced, and Listing's table36 of the distribution of the colors as the basis, we can argue out the absorptions of the filters as follows :
Color Wave-length
End 8190
Brown Middle 7680
End 7230
Red Middle 6830
End 6470
Orange Middle 6140
End 5850
Yellow Middle 5590
End 5340
Green Middle 5120
End 4910
Cyan blue Middle 4730
End 4550
Indigo Middle 4390
End 4240
Violet Middle 4090
End 3960
Lavender Middle 3840
End 3720
As it is the shadows, or clear parts of the negatives that print, therefore, these must represent the colors with which we print. It is clear that the minus-red, or green filter, should produce more or less clear glass where the red is to print. Therefore, assuming that the indigo and violet are produced by the superposition of some red, the limits of this filter must be those wave-lengths where the red is not printed, and from the above table this will be seen to be from 4550 to 5850. The transmission of the green filter, as defined by Newton and Bull, and which is usually accepted as correct, is from 4600 to 6000. In the case of the yellow, or minus-blue ink, we have another factor to take into consideration, and that is that the red ink is always of a more or less pink or magenta type, that is it reflects some of the blue and violet. To kill this and produce a full red, yellow must be superposed, therefore, the limit of this blue filter should be 4910 to 4000. The usually accepted limits are 5000 to 4000. In the case of the minus-blue or red filter, obviously the limits should be 5340 to the red end, but the region 5340 to 5850 is yellow, and therefore, blue must not print in this region, so that the absorption should be confined to these limits. Unfortunately the luminosity of the region from 5340 to 5850 is so great that it reflects also the region 5340 to 4910, and this would be recorded, hence it must be cut out, so that the absorp