The following are some examples of mixtures with which the paper or fabric is treated: -

1. - Toluoldiazosulphonate of soda...... 25 grm.

Beta-naphthol ., .. 25 „

Caustic soda .. .. 8 „

Water ...... 1000 „

2. - Ditolyltetrazosulphonate of soda.. .. 25 grm.

m-Phenylendiamine.. 8 „

Water ...... 1000 „

3. - Ditolyltetrazosulphonate of soda.. .. 25 grm.

Resorcin...... 22 „

Caustic soda .. .. 16 „

Water ...... 1000 „

The following examples will illustrate the application of ditolyltetrazosulpho-nate of soda mixed with resorcin and alpha beta-naphthol respectively, and phenylendiaroine.

Preparation of the Solutions.

1. - Ditolyltetrazosulphonate of soda ...... 30 grm.

Resorcin ...... 20 „

Caustic soda...... 15 „

All, finely powdered, are dissolved by gentle heat in one litre of water.

2. - Ditolyltetrazosulphonate of soda ...... 30 grm.

Alpha-naphthol .. .. 25 „

Caustic soda...... 7 „

Dissolved in one litre of water.

3. - Ditolyltetrazosulphonate of soda ...... 30 grm.

Phenylendiamine . .. 20 „

Dissolved in one litre of water. The solutions 1 and 2, or 2 and 3, may be mixed in equal parts. The paper is impregnated with the above mixture, and then exposed for 10-15 minutes to direct sunshine. After exposure, the picture is washed with very dilute hydrochloric acid, then with water, and finally dried.

In Lippmann's process the sensitive materials, the developers, and the fixing agents are similar to those which have been in use for many years. Lippmann does not introduce any radical chemical change; he merely modifies the physical conditions of ordinary photography, and any one who possesses a general acquaintance with the common laws of light ought to have no difficulty whatever in following out the modus operandi of colour-photography.

The essential conditions that must be present in order that colour effects may be successfully produced upon the sensitive plate are two - namely, the presence of a reflecting surface situated at the back of the photographic film, and the absolute continuity of this film; that is to say, the absence of any granules in it.

Whatever the sensitive agent employed may be (silver iodide, bromide, &c), it must be so uniformly distributed within the film of gelatine, albumen, or some other transparent or inert material like collodion, that no granules are formed that can be recognised even by means of a powerful microscope. Or, if any granulation does take place the dimensions of the granules must, in order to secure success, be so small that they are insignificant and negligible in comparison with the length of the waves of light.

Great care appears to be necessary in preparing the sensitive plate in order to obtain this essential condition of continuity.

When common albumen plates are used, Lippmann recommends that the coating be somewhat thicker than is usually thought necessary, while, if "Taupenot" plates are resorted to, a double or even a triple layer of albumen is required.

The best success has hitherto been obtained with silver gelatino-bromide. Lippmann uses the following formula: - Pure gelatine, 10 grm.; potassium bromide, 1.5 grm.; distilled water, 100 grm.

The potassium bromide and the gelatine are dissolved in the water, then filtered, and the solution is spread upon the glass plates to the required thickness.

When the plates are quite dry, they are sensitised for. at least 5 minutes (a longer time is better) in a solution of silver nitrate, containing 20 per cent. of this salt and a little acetic acid; after that they are washed thoroughly, and carefully dried. They are then ready for use.

In making a heliochrome, the sensitised plate is put into a hollow frame, or trough, into which mercury can be placed in such a way that it forms, as it were, a background to the sensitive film, or, in other words, a bright reflecting layer in close contact with the film.

Fig. 101 is a view of one of these frames fitted up ready for exposure. Here a is the sensitive plate placed in direct contact on the film side with mercury b. A piece of glass forms the back of the frame. The sides are closed in by a U-shaped piece of rubber c, which is rectangular in section. The component parts of the frame are all held tightly together by means of four strong clips d, three only of which are shown, as a portion of the front of the frame is supposed to be cut away to show the interior.

Fig. 101.

Photography In Aniline Colours Part 3 10086

Lippmann has chiefly been successful in taking photographs of spectra, although he has also taken some stained-glass windows of simple pattern.

The exposure, the developing, and the fixing operations are performed as if it were intended- to obtain a black negative; but the result is different, for when the proof is finished and dried, the proper colours of the object photographed duly appear.

The proof obtained is negative when viewed by transmitted light; in other words, each colour present in the object photographed is represented by its complementary colour. When viewed, however, by reflected light, the proof is positive - that is to say, the image appears to represent the actual colours of the object.

Lippmann has chiefly used slow plates that are not orthochromatic, and the time of exposure has varied from 1/2 hour, to about 2 hours. This long period of exposure is necessary in order to give the red rays a chance of exerting their full effect.

The process of developing is performed with pyrogallol and ammonium sesquicarbonate, in a manner similar to the method of developing "Taupenot" plates.

After being treated a sufficiently long time with the "developer," the plaits are thoroughly washed with water, then with a very dilute solution of common salt, and finally fixed with ordinary hyposulphite.

When this treatment is carefully followed out, Lippmann states that the plates are quite stable, and even when exposed to the action of the strongest electric light the character of the colours is unchanged.

The theory of this process of colour-photography is very simple; it depends upon the well-known phenomena of the interference of light.