graphiques aux encres grasses," and A. Martin's "Hundbuch der Emailphotographie und der Phototypie oder des Lichtdruckes."

The great difference between the photocollotype processes and lithography is, that whereas the lithographic stone receives a like quantity of ink in all parts of the image, and is incapable of producing a true and continuous gradation of shade, the moist gelatine film possesses the valuable property, not possessed by the stone, of receiving a greater or less amount of ink in different parts of the image, in exact proportion to the intensity of the action of the light upon them, and is thus capable of reproducing the most delicate gradations of shade as perfectly as they are shown in an ordinary silver print.

It will thus be readily understood that instead of the advantages of photographic reproduction by cheap and speedy mechanical processes being confined to the reproduction of certain special subjects, they can be extended to all classes of subjects, such as photographs from nature, brush-shaded and coloured maps, MS. records, drawings and paintings of all kinds. Even for line subjects, the process surpasses most of the known processes of photoengraving, photozincography or photolithography in the delicacy, sharpness and clearness with which the finest lines can be reproduced, as well as in perfect accuracy of scale, owing to there being no intermediate process of transfer, with its attendant washings and pressings, and the plate being printed by vertical pressure.

The process has the further advantage that the prints do not require mounting, and this makes it very suitable for book illustration, for which, indeed, it is now being very largely used. It is especially valuable for illustrations of a scientific character in cases where otherwise only the highest class of lithography or engraving would be applicable and at an enormously increased expense.

For the most successful application of the photocollotype processes to the reproduction of maps, the result depends, as in photozincography, very much on the quality of the negative, and that again on the original.

Any negative that will give a good photographic print will answer, but the successful reproduction of shaded maps or drawings demands considerable care in the execution of the original drawing as well as in taking the negative. The precautions to be taken in these respects have already been indicated in sections II and III.

For some years past my attention has been given to the utilisation of this valuable process for the reproduction of maps and other photographic work which the Surveyor General's Office is called upon to do for various Government departments. In the Proceedings of the Society for November 1871, I described a process which I had found to answer well for line work, and strenuous efforts were made to bring this and other methods into prac

tical working. It is much to be regretted that owing to the many difficulties met with in manipulating the gelatine films in the hot damp climate of Calcutta, and in getting printers with the special artistic skill required to produce the best results, our efforts have not been quite successful, and, as photozincography is found more convenient for most of the work passing through the office, the photocollotype process has not been brought into general use.

As the process previously described in the Proceedings has since then been modified and is, I know, exceedingly good for line work, the following description of the manipulations, extracted from the Annual Reports of the Surveyor General's Office for 1871-72 and 1872-73, may prove of interest, especially as the working details of few of the other processes have been published.

The printing plates are of plate glass, about or half an inch in thickness, evenly ground on one side with fine sand. When required for use they are thoroughly cleaned to remove all grease, and then carefully levelled. The composition of the gelatine coating is as follows:

c.

{

Tannin,

1 ounce.

10 grains.

Water, (in hot weather, Spirits of wine,)... 1 ounce.
The above quantity will be sufficient for two square feet of plate.

As soon as the gelatine in solution A is quite dissolved, B is added and then C is poured in gradually with constant stirring. The whole is strained through two thicknesses of cotton cloth and poured evenly over the plates on the ground side, any air bubbles being carefully removed. The plates

are then covered over with a light paper cover, to prevent dust falling on them, until they are set, when they may be removed into the open air and turned face downwards to dry. Or they may be dried with gentle heat in a drying box, but too quick drying is to be avoided because the gelatine films will dry unevenly.

When the plates are dry, they may be put away till required or sensitised in a bath of

Bichromate of Potash,.

Water,

1 part.

20 parts.

They are allowed to remain in this for 5 minutes, then removed to a drying box and dried with a gentle heat. When dry, the deposit at the back of the plates, and any inequalities at the corners of the gelatine film are * 30 grains of carbolic soap may be used instead of the albumen.

removed, and the plates are ready for exposure under the negative, which must be a reversed one obtained as described in section III.

If the reversed negative has been taken direct on glass, the exposure to light is performed in a pressure frame, in the same way as for ordinary photographs. It is advisable, however, to secure clean margins by shielding the borders of the negative by means of a mask, cut out in yellow or brown paper, which should well overlap the edges of the printing plates. The sensitive plate may be rubbed over with a little powdered soapstone to prevent any adherence to the negative. Some sheets of dark-coloured paper or cloth should be placed behind the sensitive plate and then a thick sheet of glass to give a good even pressure.

If, however, the negative has been stripped from the glass and is in the form of a thin skin, the most perfect contact will be produced by transferring the negative on to the surface of the printing film, in such a manner that it may be removed again after the exposure.

This operation presents some difficulties, but I have found the following method answer well. The sensitised and dried gelatine surface of the printing plate is covered with a very thin even coating of wax dissolved in turpentine or benzole. The plate is then placed in a dish containing sufficient spirits of wine to cover it. The thin negative film is laid down upon the gelatine in its proper position, the plate and film are then removed from the spirit, and the negative film carefully squeegeed into close contact with the gelatine surface. The plate is then covered with a few thicknesses of blotting paper, under a thick glass plate, and allowed to dry. When dry, the plate is ready for exposure. After exposure, the negative film is removed from the gelatine surface; and, if sufficient wax was used and the film is fairly tough, it comes away without tearing. Should it tear, it should at once be dissolved off with ether, or there will be a continuating action of light on the parts of the gelatine surface protected by the negative film, so that they will print darker than the rest of the plate. Before printing, the wax should be removed from the gelatine with turpentine. The object of effecting the transfer in a bath of spirits of wine is, that neither the gelatine, wax, bichromate of potash or negative film are in any way affected by it.

The duration of the exposure to light varies from 10 minutes in the sun for a clear line subject, to from 25 to 50 minutes for a subject in half tones, according to the density of the negative and the intensity of the light. It is almost impossible to judge of the progress of the printing by inspection, and it is necessary to use an actinometer as a guide to the exposure, The following form of actinometer has been found to answer well for the purpose. It consists principally of a box, in the lid of which is fixed a translucent scale divided in 14 squares of different densities, No. 1 being

almost transparent, while No. 14 is almost quite opaque; and numbers corresponding to the densities are painted in opaque colour on the scale. The scale is made by taking a collodion negative of a drawing shaded in tints of different strengths, and should be intensified so as to correspond in density with the kind of negatives it is intended to be used with.

The body of the box contains a block for carrying the sensitive surface, which may be spread on paper or on a glass plate, and a strip of vulcanised rubber below it presses the block into close contact with the scale.

I prefer to use in the actinometer a sensitive film of the same composition as the printing plate; small slips of glass are therefore coated with the gelatine mixture, sensitised, dried and exposed to light at the same time and in the same manner as the printing plates, and thus the progress of the action of light can be watched and timed very closely.

When the exposure to light is considered sufficient, the printing plate is removed from the pressure-frame and laid, gelatine side downwards, on a board covered with black cloth. The back, or under surface, of the gelatine is then exposed to light, for about 10 minutes, to thoroughly harden the gelatine and prevent it from swelling too much in the after processes. It is well to conduct this second exposure under a piece of ground glass, in order to prevent any scratches that may be on the back of the glass from showing as white lines in the print. The edges of the plate are then protected by strips of paper coated with solution of india-rubber, and when the india-rubber is dry, the plate is soaked in water until all the soluble bichromate has been removed, and is then ready for printing.

The plates can be printed in a lithographic press, but then they require to be fixed on a level stone with plaster of Paris. It has been found, however, more convenient, and in some respects better, to print them with vertical pressure in an ordinary Albion platen press; and in order to prevent the glass being broken, the bed of the press is fitted with two or three thicknesses of kamptulicon, besides a sheet of vulcanised india-rubber on which the plate rests. It is also desirable to place a piece of white paper over the bedding in order to enable the state of the plate when it is being inked up, to be better seen.

The inking in requires great skill and care on the part of the printer and is the most difficult part of the whole operation. The plate having been well soaked in water is laid on the press, and after being wiped to remove the excess of moisture, is inked in, if a line subject, with an ordinary lithographic roller charged with an ink composed of lithographic chalk ink thinned with a little olive oil, followed by rolling with a smooth roller to clear away the superfluous ink; a mask of the required size is laid on the plate to preserve the margins clean; over this comes the printing paper covered with a piece of soft felt, to drive the paper well into the hollows of

the plate; the tympan is lowered and the impression pulled in the ordinary way. The plate is then damped and inked in again, and so on.

Half-tone subjects are treated in the same manner, but it is sometimes advisable to use two kinds of ink of different consistence or depth of colour; a stiff or dark ink gives force to the shadows, while a thin or lighter coloured one will bring out the delicate half-tones. Rollers made of gelatine, glycerine and castor-oil may be used with advantage, as they drive the ink better into the hollows of the lines than the leather rollers. Capt. Abney, who has given great attention to these processes, says that the great secret of producing good results is to have the command of first rate rollers. Glazed enamelled paper is generally used for printing half-tone subjects, but in some cases unenamelled paper answers well. The most suitable paper for printing seems to depend partly on the composition of the sensitive surface and partly on the ink.

One of the great drawbacks to the extended use of the photocollotype process for the reproduction of maps is the difficulty of making corrections. on the plates. When the printing surface is a metal plate or lithographic stone, upon which a map has been either engraved, zincographed or lithographed, additions and erasures may easily be made without any risk of the loss of the printing surface or even of much damage to it. With the tender gelatine films the case is different, and although writing or simple lines may be inserted without much difficulty, it would be almost impossible to successfully alter gradation of shade or to insert shaded details. On the other hand, the taking out of details must be done by some chemical means which must always be attended with the imminent risk of raising the gelatine film from its support and the consequent utter destruction of the printing plate.

As maps, almost more than any other printed subject, require that it shall always be possible to make corrections on the printing plates, it is obvious that the use of any process which will not permit of this being done must be confined more to the reproduction of maps already printed or of an ephemeral character than to the preparation of new or standard ones. And thus, though photocollotype is admirably adapted for reproducing copies of old or other special maps, which are, or can be, finished once and for all, it is not suited for maps on which corrections are likely to be required.

With the plates prepared as described we have found that details may be inserted by two or three methods. The first is by writing in the required additions on the dry gelatine surface, using an ink composed of bichromate of potash, either alone or coloured with Indian ink. After the insertion of the additions the plate is exposed to the light for a few minutes to reduce the bichromate, and may then be washed and printed as usual.