As the light falls upon the uncovered part of the diazo-primulined silk, it destroys the colour, and when that action has proceeded sufficiently far, the silk is taken out of the frame, the image of the fern-leaf will now be seen in a faint yellow colour upon the surface. In the bright lights the diazo-compound will be completely - in the half lights only partially - decomposed, so that a perfect reproduction of the original is obtained in the form of diazo-primuline.

While the printing process is proceeding, the developing-bath can be prepared. It consists, for this particular colour (say purple) of a solution of alpha-naphthylamirie hydrochloride containing 1/4 per cent.; but the picture can be developed with any of the various amines or phenols, which form a dye with the diazo-compound, if any other colour is wanted.

Into the developing-bath the pre-pared silk is plunged, and immediately pushed well down beneath the surface with one of the glass rods. In a few seconds the image of the fern-leaf will become changed from a faint yellow to a rich purple hue.

The whole process is now completed by well washing the material in cold water.

The development of colour corresponding to the shadows and half-tones of the object photographed is exactly proportionate to their depth, to the degree of protective action which they have exerted on the sensitive surface beneath - in other words, to the quantity of diazo-primuline which survives the decomposing influence of the light.

Should several leaves or flowers form the pattern, each can be developed in a different colour, after printing, by applying different solutions of amines or phenols to the different flowers or leaves by means of camel-hair pencils. Hence, in some cases, even something like the natural colouring can be followed.

Another way in which this photographic process can be utilised for decorative purposes is in making glass transparencies in various colours. Here the operations are perhaps slightly more difficult, in so far as it is necessary, in the first place, to coat a piece of glass of the required size, and which will fit the frame, with a thin film of gelatine, which has previously been mixed with a little of the primuline. When the film is dry, the prepared glass is treated exactly in the same way as in the case of the silk, except that the time for which it is left in the baths is necessarily more prolonged to enable the solutions to thoroughly saturate the gelatine. As we have already seen, nitrogen gas is given off as the diazo-compound decomposes under the in-fluence of light, and unless the gelatine be sufficiently well soaked the tiny bubbles of gas will be entangled.

The prints obtained in this way are "positive," the lights and shadows in the object being faithfully reproduced in their true value in the coloured . picture. Natural objects of convenient size and form may therefore be photographed directly. Reproductions from camera-pictures require glass positives or positive paper prints, such as over printed bromides, made transparent in the usual way with vaseline.

If only one solution is used in developing the picture, it will, of course, be a "monotint," just like any other picture by any printing process would be.

In the case of the primuline prints, one feature, apparently unavoidable, is the more or less yellow background which they possess; but it is possible to get prints with other bodies of this class, which give a very satisfactory approximation to a colourless background.

A primuline print is more nearly a measure of the visual intensity of the sun's rays than the sensitive films used in the ordinary methods of photography, in relation to which it is well known that the photographic intensity of sunlight is a very different thing to the visual intensity, owing to the enormously greater activity of the blue and violet rays in decomposing the compound which they employ.

The experiments have already brought to light the following new and important facts: -

1. The action of the light consists in the decomposition of the diazo group, with evolution of nitrogen, probably with the formation of the corresponding primuline phenol.

2. The rapidity off the action of the light varies, other things being equal, with the nature of the substance with which the diazo compound is combined.

3. Photographic reproductions of the spectrum show that as regards intensity of action the various rays of light are not in the same order as that in which they stand with reference to silver bromide, chloride, etc.

Taking into consideration the extreme simplicity of the process, the cheapness of the materials and apparatus, the fast nature of the colours, the scope it affords for artistic taste, and the almost endless purposes for which it can be used, it should be widely adopted. (Eng. Mech.)

Another process has been patented by Dr. Feer, and the invention is based on the fact that diazo-sulphonic salts (R - N = N - SO3Na) with phenol alkali, and chlorides of or free aromatic [amines, react under the influence of solar or of the electric light, forming an azo dyeing substance. For carrying out the process, the inventor impregnates paper or textile fabric with a dilute molecular mixture of a diazo-sulphonic salt (for instance, of aniline, amidoazobenzole, benzidine, and their homologues) and phenol alkalies (for example, phenol, resorcin, and beta-naphthol) or chloride of or free amines (aniline, naphthylamine, phenylendia-mine, and homologue). The paper or fabric is then dried in the dark, and' exposed for about 5 minutes to the sun, or to the electric light. Thereby is formed in the illuminated portions an insoluble azo dye, while the parts protected by the opaque portions of the negative remain in their original colourless and soluble condition. The picture is thus developed while printing. It is, after exposure, washed with water, or with very dilute hydrochloric acid, whereby the unaltered sensitive preparation is washed from those parts not affected by light through the negative. The picture is thus fixed, and only requires drying to finish it.