Photography in Natural Colours
logues, price lists, etc., are too well known to need any descrip-tion. Its uses in astronomy for making charts of the celestial bodies, in the interests of justice for the detection of criminals, for the purposes of experimental warfare and as an assistant in scientific research, for sounding the depths of the sea and for pathological study in medicine, seem almost unlimited.
Photography in Natural Colours. The methods of repro-ducing objects in the colours of nature may practically be divided into three heads: (i) by the use of subchloride of silver; (2) by Lippmann's process, or the production of interference waves ; and (3) by the three-colour sensation process. The first process is that which was experimentally studied by Seebeck, Becquerel, Niepce de St. Victor, Poitevin, Kopp, etc., and practically con-sists 01 exposing subchloride of silver to the coloured rays, either of the spectrum or beneath a coloured glass positive, etc. The\disadvantage of this process is that there are no very satis-factory means of fixing the colours which are thus obtained, and consequently the results will not bear a prolonged exposure to light. Some of the most successful results can be obtained by using commercial gelatino- or collodio-chloride paper, and treat-ing it as suggested by Kopp. The paper should be exposed to diffused light whilst immersed in the following solution till it turns blue-green : -
Chloride of zinc ... ... ... ... 3 grs.
Sulphuric acid ... ... ... ... 2 drops.
Water ......... ...... 5 ozs.
The paper can then be well washed and dried between blotting paper and kept in the dark. It will keep in this state for some time. The following solution should be prepared : -
Potassium bichromate (pure) ... ...oz.
Cupric sulphate (pure) ... ... ...,,
If necessary the ingredients should be dissolved with heat. Mercurous nitrate,oz., should also be dissolved in as small a quantity of water, acidulated with nitric acid, as possible ; the two solutions mixed, both being at 212o F.; the mixture fil-tered, and the total bulk of the solution brought up to 100 c.cm. The blue-green paper is bathed in this for a minute, well drained,