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Color Filters or Color Screens 73
With this filter the whole of the ultra violet passed by the lens and to which plates are sensitive, is allowed to act, and the eosin subdues the bluegreen spectrum. The result is to strongly differentiate the yellow-printing plate from the others. With most subjects the advantages of this extra differentiation more than counterbalances the disadvantages. Should it be preferred to work without the ultra-violet rays, the following should be used :
Eosin, solution C 2.1 ccs.
Potassium chromate, solution A 10.5 ccs.
Distilled water to 1000 ccs.
If the effect of the blue-green is to be increased the eosin is omitted or reduced in quantity. It will be observed that by increasing or diminishing the quantity of eosin or yellow chromate respectively, the spectrum action can be shifted laterally in either direction, and either concentrated in one band or spread over a considerable area. Filter for red-printing plate :
Potassium chromate, solution A 31.25 ccs.
Methylen blue, solution D 11.50 ccs.
Distilled water to 1000 ccs.
With this filter the aim is to obtain the maximum effect in the greens, combined with equal action in the yellow-greens and blue-greens; this being necessary owing to the general lack of green luminosity in natural objects and pigments. If desired, more or less blue-green can be allowed to pass by weakening the yellow chromate or vice versa. If more or less of the yellow end of the spectrum is desired the methylen blue is weakened or vice versa.
Filter for blue-printing negative :
Potassium dichromate, solution B 1000 ccs.
Eosin, solution C 26 ccs.
Ammonia, sp. gr. 0.880 26 ccs.
The ammonia is to redissolve the precipitate formed on addition of the eosin to the dichromate solution. With this filter the whole of the spectral red rays act. The other limit is practically at the D line, the action ending abruptly at this point. Should it be preferred to carry the action further towards the green the eosin can be reduced or omitted altogether. The following formulas were given by Klimsch49 for use with collodion emulsion (see page 236) :
Violet filter :
Methyl violet, 0.5 per cent sol 250 ccs.
Distilled water 1000 ccs.
Green filter :
Picric acid, 5:1000 sol 210 ccs.
Carmin blue (Hoechst), 1 :1000 sol 80 ccs.
Distilled water 1000 ccs.