Photography by the Interferential Method. 11 



exposure. It is then developed and fixed by the usual means em- 

 ployed in photography, the result being a fixed colour photograph 

 visible by reflected light. 



The mirror is easily formed by means of mercury. The glass plate 

 carrying the film being inclosed in a camera slide, a quantum of 

 mercury is allowed to flow in from a small reservoir and fill the back 

 part of the slide, which is made mercury-tight. The plate is turned 

 with its glass side towards the objective, the sensitised film touching 

 the layer of mercury. After exposure, the mercury is allowed to 

 flow back into its reservoir, and the plate taken out for development. 



The only two conditions necessary for obtaining colour, trans- 

 parency of the film and the presence of a mirror during exposure, 

 are physical conditions. The chemical nature of the photographic 

 layer has only secondary importance ; any substance capable of giving, 

 by means of an appropriate development, a fixed colourless photo- 

 graph, is found to give, when backed by the mirror, a fixed colour 

 photograph. 



We may take, for instance, as a sensitive film, a layer of albumeno- 

 iodide of silver, with an acid developer; or a layer of gelatino- 

 bromide of silver, with pyrogallic acid, or with amidol, as deve- 

 lopers. Cyanide or bromide of potassium may be as usual employed 

 for fixing the image. In a word, the technics of ordinary photo- 

 graphy remain unchanged. Even the secondary processes of intensi- 

 fication and of isochromatisation are employed with full success for 

 colour photography. 



The photographic films commonly in use are found to be opaque, 

 and formed, in fact, by grains of light-sensitive matter mechanically 

 imprisoned by a substratum of gelatine, albumen, and collodion. 

 What is here wanted is a fully transparent film, the light-sensitive 

 matter pervading the whole of the neutral substratum. How can 

 such a transparent film be realised? This question remained 

 insoluble to me for ma,ny years, so that I was debarred trying the 

 above method when I first thought of it. The difficulty, how- 

 ever, is simply solved by the following remark. It is well known 

 that the precipitation of a .metallic compound, such as bromide of 

 silver, does not take place in the presence of an organic colloid, such 

 as albumen, gelatine, or collodion. In reality, the metallic compound 

 is formed, but remains invisible; it is retained in a transparent 

 modification by the organic substances. We have only, therefore, to 

 prepare the films in the usual way, but with a stronger proportion of 

 the organic substratum ; the result is a transparent film. By mixing, 

 for instance, a gelatinous solution of nitrate of silver with a gelatinous 

 solution of bromide of potassium, no precipitate is formed, and the 

 result is a transparent film of dry gelatine containing 15 and even 30 

 per cent, of the weight of bromide of silver. 



