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56 History of Three-Color Photography
A
T
e
6000
1.00
0.00
5900
0.90
0.05
5800
0.67
0.17
5700
0.34
0.47
5600
0.28
0.55
5500
0.37
0.43
5400
0.54
0.27
5300
0.59
0.23
5200
0.60 •
0.22
5100
0.70
0.15
5000
0.80
0.10
In which A = the wave-length, T = the transparency and e = log (1/It). In order to find e, therefore, it is only necessary to divide 1 by T and find the logarithm of the quotient. For instance, if we take wave-length 5400 in the table, T = 0.54, then 1/0.54 == 1.844 and the log of this is 0.266.
The figures in the third column represent the extinction co-efficients for the given wave-lengths, and the height of the ordinates for a chart. As a rule the height of the ordinates is so chosen that that for a 0.1 film corresponds to about one-sixth of the length of the spectrum between C and G. For a complete discussion of these factors, see Kayser's "Handbuch der Spektroscopie," Vol. 3 and S. E. Sheppard's "Photochemistry," 1914, 136.
J. Precht and E. Stenger9 estimated the actinism of the colors in daylight by the use of additive filters, with the following transmissions : for the red, 7250 to 5700; for the green, 5800 to 5050; for the blue, 4900 to 4100. The ratio of the correct exposures with which should be 1:4:17, for the blue, green and red respectively. Exposures were made in sunlight and in the shade on cloudy days, and the actual exposures to obtain the same densities were as follows : in the sun, B 1 : G 3.2 : R 8.8 and B 1 :G2.7 : R8.1 and B 1 : G 3.3 : R 9.0 for three different conditions of light. In the shade and on cloudy days the ratios were B 1 : G 5.6 : R 14 and B 1 : G 4.0 : R 11.7 and B 1 : G 5.1 : R 23.3. It will thus be seen that under equal conditions of exposure in clear and cloudy weather at the same time of day, a strong reduction of the red action takes place. They came to the conclusion that the general assumption that the density is directly proportional to the time of exposure does not hold good in threecolor work.
They also directed attention to the influence of the duration of development, and, as was pointed out by H. W. Vogel, the yellowish-green sensitiveness is only correctly rendered with a sufficiently long development. This applies to tri-color work, though not to the same extent; although, on the other hand, the color differences are lessened as the extreme densities are reached.
Turning their attention to arc lights, it was found that the following