ABSTRACT
The name of Heliochromy, or sun-colouring, is very appropriately given to a process by which photographic pictures in their natural colours are obtained. The colours of natural objects are produced by the decomposition of the rays of light, this being effected by some peculiar surface action. The green rays are obtained with boracic acid or the chloride of nickel; also with all the salts of copper. The blue rays are obtained with the double chloride of copper and ammonia. Indigo rays are obtained with the same substance. The violet rays are obtained with the chloride of strontian and the sulphate of copper. Iodine and bromine, and their salts, have been tried, but they will not produce a surface capable of developing colours. Chlorine, in the state of chlorates or chlorides, is the only substance which possesses the property of being coloured by light, when chemically combined with metallic silver.
