BICHROMATE LAMP A lamp for dark-room use in which a solution of potassium bichromate serves as the light filter. Howard Farmer found that the various kinds of ruby and orange fabrics and glass in common use transmit only 2 per cent. or less of the light, whereas a 6 per cent. solution of potassium bichromate gives quite as much safety and gives more than 8o times the amount of illumination possible with orange glass. With other solutions the differences are still greater, but the potassium bichromate solution appears to be the best for general use. Most bichromate lamps are based on the pattern designed by Farmer, whose original lamp is shown at A. It consists of two concentric glass cylinders, about 4 in. and 5 in. in diameter respectively, placed one inside the other, mounted on a suitable solid base and furnished with a wooden cap, in which is mounted an incandes cent electric bulb. This is excellent as a central light. A glass tank, for use with oil or gas lamps,
is shown at B and C, such tanks being filled with a suitable solution and used in place of the usual red glass. As either type of lamp may be filled with any light-filtering solution, an oppor afforded of adapting the actinic quality of the light to particular requirements. A 6 per cent, solution of potassium bichromate is safe for bromide papers, but not for dry plates, especially isochromatic plates, a safe solution for which is made as follows : Dissolve i oz. of the bichromate in about g oz. of water. Take about 4 oz. of the solution, and add i drm. of eosin, which is a strong red dye, and gently heat until the colour is a deep red ; mix the two solutions and pour into the lamp cell. Should the solution have a muddy appearance, pass it through filter paper.