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Color Filters or Color Screens 57
densities were obtained with increasing watts, using the same filters as above :
Watts
Blue
Green
Red
400
1.0
1.01
0.84
700
1.0
0.72
0.68
1900
1.0
0.68
0.64
It is obvious from these results that with altered conditions of light the reproduction of colors must suffer considerably, when the exposures given are according to the filter ratios for ordinary daylight.
The Making of Filters. — It may be as well to point out that except under special circumstances, it is hardly worth the average worker attempting to make his own filters, considering especially that gelatin film filters can be obtained at very reasonable prices, and so perfectly and evenly coated, that it is almost hopeless for the amateur operator to try and emulate. For those who want to experiment there are one or two books, that are quite indispensable, unless mere trial and error is adopted. These works are "An Atlas of Absorption Spectra," by H. S. Uhler & R. W. Wood, Washington, 1907, which comprises the absorptions from the red, about 6200, to the ultra-violet, at 2000. But it should not be overlooked that as the authors used a Nernst lamp, and an alloy of cadium and zinc for the ultra-violet, the strong lines of this alloy are recorded, when the normal light of the said wave-lengths would not be transmitted. Nor must one forget that in the average photographic work, one has not only the glass of the lens, but that of the filter also, and these practically cut out the ultra-violet below 3000 at most. C. E. K. Mees has also published "An Atlas of Absorption Spectra," 1909, which shows the absorptions of many dyes in various strengths, from 3500 to 8000. From this work especially it is comparatively easy to determine the dyes necessary to use for any desired filter. An equally valuable work, if not more so to the practical worker, is "Wratten's Light Filters," issued by the Kodak Co., which gives the absorptions of all filters made by this firm, von Hubl's "Die Lichtfiiter," 1922, is specially valuable also. For out-of-the-way dyes, that may be rarely used in photography, one should refer to H. Kayser's "Handbuch der Spektroscopie," 1903, Vol. 3, and J. Formanek's "Spectralanalytische Nachweis kiinstlicher organische Farbstoffe," Berlin, 1900. With reference to the commercial names and chemical terms and makers of dyes, "FarbstofTtabellen" by G. Schultz, Berlin, 1923, is excellent, but to be preferred is "Colour Index" by F. M. Rowe, Bradford, 1924.
The use of collodion as the vehicle of the filter dyes is not to be recommended, for it is difficult to obtain an absolutely even film, and it is by no means easy to allot a given amount of dye to a given area. Further than that, one is more limited in the choice of dyes, as obviously they must be soluble in alcohol-ether or like solvents.
In order to avoid striae, or wavy lines, J. Waterhouse10 recommended