Printing the transparencies may be effected either in the camera or by superposition, the latter being the method usually followed; but if the negatives be a different size to that of the required slide, the camera must be used, and with it a lens capable of giving good definition with a large aperture. For, compared with wet collodion, the albumen process is slow, and in comparison with gelatine very slow indeed, although it is not so slow as the gelatine chloride process. With the camera the exposure will necessarily be somewhat long, and, as a rule, when prolonged exposures have to be given, the colour of the image is rarely very satisfactory.

In printing by superposition, either diffused daylight or artificial light such as a gas flame or a paraffin lamp -may be employed. With regard to the time of exposure little can be said, as all will bo dependent upon the source of light employed, the distance the plate is placed from it, as well as upon the density of the negative itself. Therefore, the experimentalist must determine this matter for himself. This he can easily do by exposing a plate or two under a negative of average intensity, giving different times for different portions of it, and then developing. One or two plates exposed in this way will enable a very correct judgment to be formed as to the exposure required in future for every class of negative. When once this is arrived at it remains constant, because, unlike the gelatine, each batch of albumen plates prepared may be relied upon as being of equal sensibility.

We now come to the development of the image. This at one period was treated, and preserved, as a great secret. The developing solution, after all, is very similar to that used in the wet-collodion process before the iron developer was introduced, except that it contains a little citric acid, and that it is employed warm. A good developer is as follows:-

Pyrogallic acid . 25 gr.

Glacial acetic acid. 1/2 oz.

Citric acid .. 10 gr.

Water .... 10 oz.

This solution had better be made and kept in a Florence flask, so that it can be heated and kept warm over a spirit lamp when required for use.

The exposed plate is first placed in a dish of distilled water, heated to about 150° F. until it has acquired that temperature. It is then removed, slightly drained, and flooded with the developing solution, which has previously been heated to about 130-140° F. Immediately before the solution is applied, it must have about 4 drops per oz. of a 5 gr. solution of nitrate of silver added. If properly exposed, the image will quickly appear, and by the way it comes out it may be judged if the exposure has been rightly timed or not, similarly as in the development of plates by any other process. As the films are so very thin and transparent, the density of the image can easily be judged of by transmitted light. It is always best to err on the side of under than over-density, because, in the latter case, the slides will always appear dense and heavy on the screen; whereas if they be slightly thin it may, to some extent, be remedied in the toning. As a guide in the development, it may be borne in mind that the more fully the plates are exposed and the more rapidly they are developed, as also the less silver employed in the developer, the warmer will be the colour of the image; while the slower the development, either from the solution being cold or the plate under-exposed, or if too much silver be used, the more the picture will approach an olive-brown tone.

As the development proceeds the plate must be carefully watched for stains or fog. If any appear* the plate must at once be washed under the tap, and the surface rubbed with a pledget of cotton wool, which will remove them. The development can then be recommenced with a fresh batch of solution and silver, repeating the treatment with the cotton wool if found necessary. When the development is complete, the plate must be thoroughly washed under the tap -to remove all traces of the pyrogallic acid, which, if allowed to remain, would tend to injure the toning bath.

The plate is now ready for fixing and toning. This is usually done in one bath, which is made as follows: - 1/2 lb. hyposulphite of soda is dissolved in 1/2 pint water; then 3 gr. of chloride of gold, dissolved in 2 oz. water, is added very gradually, and with vigorous stirring. After standing 12 hours and being filtered it is ready for use. It is then placed in a flat dish' and the plate is immersed. The iodide is quickly dissolved out, but the toning proceeds slowly - 1/4-1/2 hour or more being frequently required to obtain deep, rich tones. But much depends upon the colour and density of the image at starting. When the desired tone is obtained, the plate is well washed under the tap, and afterwards soaked in plenty of water, and again rinsed to ensure the entire removal of all traces of the hypo. Indeed, as much care should be bestowed on this part of the operation as in the case of gelatine negatives, in order to ensure permanency.

Alkaline gold toning (after fixing in plain hyposulphite and thoroughly washing) may be employed instead of the double fixing and toning bath: but the colour obtained has not been so satisfactory as by the method just described, which is that used by Ferrier and Soulier. (Brit. JL Photog.)

(f). By contact printing. This is capable of producing slides equal in quality to any other mode of preparation that an amateur, or one who does not make a specialty of such work, can produce, and is at the same time so simple that any ordinary amateur can obtain satisfactory results.

In the first place, no dark room is required, as the emulsion can be prepared by gaslight, and no more care is required than in the preparation and printing of albumenised paper. No washing of the emulsion is necessary; indeed, as far ns the brilliancy of the slide is concerned, it is a disadvantage. One can see how the printing of the slide is going on as well as if it were an ordinary albumenised paper print; and the latter is the greatest gain of all, because we do away with the uncertainties of the blind processes, i. e., any process in which light forms an invisible image requiring development.