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236 History of Three-Color Photography
precipitated with excess silver nitrate and the precipitate washed till the water became deep red; then dilute eosin solution was poured over the precipitate to convert any possible trace of silver nitrate, it was again washed twice with water, and finally dissolved in alcohol to which ammonia had been added, and enough was added to make the emulsion a deep rose-red color.
Klimsch79 suggested Albert's emulsions should be used for tri-color work, for the filters (see p. 73), for the violet and green the mixture should be:
Eos emulsion 1000 ccs.
Sensitizer A 70 ccs.
Sensitizer GG ' 50 ccs.
Allow to remain a few minutes before coating, wash the plate well before development. For the orange filter the following should be used :
Ethyl violet, Hoechst, 2 : 1000 ale 25 ccs.
Acridin orange NO, 2 : 1000 ale 30 ccs.
Alcohol 45 ccs.
Emulsion 1000 ccs.
Shake well, filter and allow to stand a day before use. When the coated plates have set they should be immersed in :
Gum 25 g.
Glycerol 90 ccs.
Tannin, 1 : 1000 ale. sol 5 ccs.
Water 1100 ccs.
The plates should be well washed before development.
Abney80 published a method of making a red-sensitive emulsion without the use of dyes. His initial idea was to make an emulsion that should be transparent for the shorter wave-lengths and opaque to the longer, or in other words, sensitive to the longer. His first results made by the addition of resins were not successful and he then adopted the following method: "A normal collodion is first made according to the formula: — pyroxylin (any ordinary kind) 16 grains ; ether, sp. gr. 0.725, 4 ozs. ; alcohol, sp. gr. 0.820, 2 ozs. This is mixed some days before use, and any undissolved particles are allowed to settle, and the top portion decanted off; 350 grains of pure zinc bromide are dissolved in fA to 1 oz. of alcohol (0.820) together with 1 drachm of nitric acid. This is added to 3 ozs. of the above normal solution, which is subsequently filtered. Then 509 grains of silver nitrate are dissolved in the smallest quantity of hot distilled water, and 1 oz. of boiling alcohol added. This solution is poured into the bromized collodion, stirring briskly while the addition is made. Silver bromide is now suspended in a fine state of division in the collodion, and if a drop of the fluid be examined by transmitted light it will be found to be of an orange color. Besides the suspended silver bromide, the collodion contains zinc nitrate and nitric acid and these have to be eliminated. The collodion emulsion is turned out into a glass flask