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582 THE PHOTOGRAPHIC NEWS. [September 14, 1883 a second, the duration of each exposure is but one- millionth of a second. The revolution of the disc is brought about by a heavy weight, which is show.n better in fig. 3, the cord C being wound up, first of all, by the winch, and then permitted to run down. There is, inside the carriage, a break and regulator to act upon the disc when in movement. Fig. 8 also shows something of the interior arrangements of the moving laboratory. The camera, which is seen at A, is capable of taking plates of sufficient length to include the whole of the background, and thus receive images of the moving model as this progresses across the field of the lens ; B is the revolving disc, which we have already seen in fig. 2 ; and D is a shutter, which is lifted vertically at the commencement of the experiment, and lowered at its close, to prevent any extraneous light from entering. E is a large opening through which the object is photo graphed, serving to shade the lens from injurious sun light. The darkness that reigns in the little laboratory obviously permits the photographer to work with great ease and certainty. THE WOODBURYTYPE PROCESS. First Article. Before proceeding to give such practical details as shall place success in Woodburytype printing within the reach of any careful person, we will briefly outline such points as are necessary for a proper comprehension of the princi ples upon which the Woodbury process is founded. The so-called carbon print is a photo, relief of tinted gelatine, its maximum of thickness being in the deep shades, and this relief is ordinarily attached to the surface of a sheet of paper or a plate of glass. The high lights of the picture are those points where the layer of gelatine is either altogether wanting, or possesses a minimum thickness. A carbon print is obtained by exposing a film of tinted gelatine, made sensitive by means of an alkaline bichromate, to the action of light under a negative. The tinted gela tine is thus rendered insoluble to a depth corresponding to the action of light, and all unaltered gelatine is now washed away by means of warm water, leaving the relief or picture in tinted gelatine. An essential feature of the carbon pro cess is, however, the washing away of the unaltered gela tine from that side of the sensitive film which was farthest from the negative during exposure, and Fargier attained this end by spreading the sensitive mixture on collodion, and exposing through the film. He thus obtained a gela tine relief supported upon a basis of collodion. The pro duction of such a relief forms the first step in the Woodbury- type process, and from this relief a mould is taken, generally of lead. In this mould any number of fresh pictures are cast in tinted gelatine. Viewed from this point of view, the Woodburytype process may be regarded merely as a method by which carbon prints may be multiplied by the simple process of casting. As, however, the mould is taken from a dry relief, and the cast is made in wet gela tine, it is obvious that this cast must, in drying, shrink down to very much less than its original thickness. The practical result is, that the original print from which the mould is made must stand out much more boldly in relief than an ordinary carbon print. One may assume one two-thousandth of an inch as somewhere about the thick ness of the gelatine of a representative carbon picture ; but a mould from this would be quite useless for printing purposes. To produce a gelatine cast which, when dry, shall, like an ordinary carbon print, have a thickness of about one two-thousandth of an inch, it is necessary to use a mould having a maximum depth equal to about ten times this, or one two-hundredth of an inch in thickness. It will thus be seen that the gelatine relief, from which the Woodburytype mould is made, differs from an ordinary carbon print by being about ten times thicker, and the consequence is, that the ordinary difficulties of carbon printing are considerably exaggerated in making the reliefs. In the case of the usual carbon print, the gelatine is ordinarily tinted with a considerable proportion of very finely-divided lamp black or other pigment, and the presence of this pigment serves to so far block the progress of the rays of light as to prevent the production of a very high relief; but in making the original print or relief for Woodburytype, the amount of pigment is reduced to a minimum, or sometimes omitted altogether. Carbon printers find that if their tissue is allowed to dry very slowly, the bichromated gelatine becomes insolu ble, just as if light had acted on it; and this liability to become insoluble would become a serious difficulty in dealing with the thick tissue employed for Woodburytype, were it not for the circumstance that it is possible to dry the sensitive gelatine very rapidly by means of a special cupboard containing chloride of calcium. Before giving details as to the means of preparing the tissue, we will describe an improved drying cupboard which we have found very useful, not only for Woodburytype tissue, but also for many other photographic purposes. The degree of heat applied by the lamp must be very slight, and the interior of the drying box should never attain a higher temperature than 30° Centigrade or 86% F. A A is a tin box or cupboard, with’shelves placed alter nately like baffle-plates; B, B, B, B, movable trays or dishes containing dried calcium chloride ; C, C, C, C, the coated.glass plates, resting in grooves, so that the gelatinous coating shall be directed downwards towards the calcium chloride; D, bent tube, which is so heated by the lamp, E, as to cause the same air to circulate continually in the direction indicated by the arrows ; F, F, dotted space showing position of chloride of calcium box in another form of the apparatus. That form of the apparatus in which the trays contain ing chloride of calcium are replaced by a receptacle in the place indicated by dotted lines (F I'), is especially effective, as in this case the air is desiccated just before it is heated by the lamp, an obviously advantageous arrange ment. The drying-box ordinarily employed for Woodburytype tissue consists simply of a case with shelves, a tray of calcium chloride being placed on each tray, and the coated glass plates are placed over the calcium chloride as shown in our drawing. This arrangement makes no provision for ensuring a circulation or motion of the air, nearly twenty hours being frequently necessary for the drying of the tissue. The circulating desiccator described above is much more effectual, about six or eight hours being sufficient. In our next article we will describe the method of pre paring the sensitive Woodburytype tissue.