ORTHOCHROMATIC COLLODION EMULSION.—A collodion emulsion rendered ortho chromatic by the addition of so•called color-sensitizers, the effect of which is to produce photo graphic negatives more correct with regard to the color gradations as apparent to the eye. Mr. J. M. Eder's formula is as follows : The emulsion is prepared with— A.-15 grammes crystallized nitrate of silver dissolved in 12 water, and then 90 c.c. alcohol of 95% is added ; the whole is then mixed with t5o c.c. of 4% raw collodion.
B.-15 grammes pure crystallized calcium bromide are dissolved at a gentle heat of 75 c.c. of an alcoholic solution of (t:800) eosine, and then added to 15o c.c. of 4% raw collodion.
In the dark-room the collodion A, containing the bromide, is to be gradually added, well shaking the whole time, to the collodion containing the silver, until only about 5 or to c.c. of the bromide and collodion remain. The emulsion is then tested for an excess of silver nitrate. This is done by pouring a small quantity of the collodion on to a glass plate, and, when stiff, let fall a drop of potassium dichromate solution upon it. If a large excess of nitrate of silver be present, a spot of silver chromate will be produced. A further quantity of the plain collo dion is then added, and the test repeated. When the drop of dichromate solution gives only a
pale orange-reddish spot the emulsion is fit for use.
This collodion emulsion only works well when a slight excess of silver nitrate is present, inducing the formation of eosine of silver. If no excess be present, the emulsion will be very insensitive, while, on the other hand, if the excess be too great, the film is much more sensitive, but is liable to produce dirty and foggy images. The emulsion may be used at once, but the results are better if it be kept for twelve or twenty-four hours. The plates are coated in the usual manner, and dipped in a vessel of spring water, until the water ceases to be repelled at any part of the surface. It is best to expose while still wet ; when dry they are much less sensitive. The time required for exposure is about half that necessary for wet collodion plates with the iron developer. Most painting or colored objects may be photographed without the interposition of a yellow screen. A harsh blue may be softened down by using a screen of collodion stained with aurantia. The following will be found a serviceable developer :