Camera Work: A Photographic Quarterly (1907 (Heft 20))

have a person’s appearance depend upon the weather conditions at the time
of the taking. We must, therefore, be able to modify the quality of the
light reaching the plate so as to make it the equivalent of white light.
If this be judged too glaring, it can as will be shown later, be mellowed
to any extent.
Exception has purposely been made to the color-reproductions of fres-
coes, mural paintings, etc. These, in justice to the painter, must, if possible,
be reproduced in their setting and in the light in which they are intended to
be seen.
ATMOSPHERIC ABSORPTION OF LIGHT.
The following figures deduced from the formula given by Lord Ray-
ieigh in the Phil. Trans. of R. S. 1887, show how important are the
variations in the quality of the light at different hours of the day.
Assuming that sunlight before traversing the atmosphere is made up
of 1000 parts of each color, the amount of the different colors transmitted
through varying thicknesses of atmosphere will be as shown in the table,
the figures 1-2-3-8, being the relative thicknesses of atmosphere traversed
and 90°-30°, etc., indicating the elevation of the sun above the horizon.

90°
or
2 Atmosphere
30°
or
3 Atmosphere
I9°30
or
8 Atmosphere
7°30
or
Red at B..
925
855
795
550
Orange at C.
910
835
760
480
Yellow Orange at D.
865
755
655
325
Green at E ........
805
645
520
170
Blue at F .
735
545
405
85
Violet at H.
510
250
120
0
As will be noticed sunlight in traversing 1 atmosphere loses about 9

The above table is for normal absorption. It is clear that there is an
additional variation due to local atmospheric conditions, such as clouds, rain,
haze, etc.
And, when it traverses 8 atmospheres, a little before sunset, it has lost 45
per cent. of the red and 100 per cent. of the violet. At 32 atmospheres,
the sun being at the horizon, the yellow, the green and the blue and the
violet are entirely absorbed.
Such being the working conditions of the color-photographer, it follows
that instruments must be used to measure, quickly and accurately, the quality
of the light at the time of the taking of the photograph and must, moreover,
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that instruments must be used to measure, quickly and accurately, the quality
of the light at the time of the taking of the photograph and must, moreover,
have means, in his lens, to quickly make such corrections as may be neces-
sary to ensure faithful reproduction of an object as it would appear in
white light. (3 A. Brasseur.

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