exposure. I t 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 form ed by means of mercury. The glass plate carrying the film being inclosed in a cam era slide, a quantum of m ercury is allowed to flow in from a sm all reservoir and fill the back part of the slide, which is made mercury-tight. The plate is turned with its glass side tow ards 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, transparency of the film and the presence of a mirror during exposure, are physical conditions. The chemical n ature of th e photographic layer has only secondary im portance ; any substance capable of giving, by means of an appropriate development, a fixed colourless photograph, is found to give, w hen backed by the mirror, a fixed colour photograph.
W e may take, for instance, as a sensitive film, a layer of albumenoiodide of silver, w ith an acid developer; or a layer of gelatm obromide of silver, w ith pyrogallic acid, or w ith amidol, as developers. Cyanide or bromide of potassium m ay be as usual employed for fixing the image. In a word, the technics of ordinary photography remain unchanged. Even the secondary processes of intensification and of isochrom atisation are employed w ith 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 m atter mechanically imprisoned by a substratum of gelatine, albumen, and collodion. W hat is here w anted is a fully tran sp aren t film, th e light-sensitive m atter pervading the whole of the neutral substratum . How can such a transparent film be realised ? This question rem ained insoluble to me for many years, so th at I was debarred trying the above method when I first thought of it. The difficulty, however, is simply solved by the following rem ark. I t is well known th a t the precipitation of a m etallic compound, such as bromide of silver, does not take place in th e presence of an organic colloid, such as albumen, gelatine, or collodion. In reality, the m etallic compound is formed, but rem ains invisible; it is retained in a transparent modification by the organic substances. W e have only, therefore, to prepare the films in the usual way, but w ith 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.
