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2 THE PHOTOGRAPHIC NEWS. LJAXTARY 2, 1885. described by Mr. Cowan is better adapted for coating a considerable number of sheets in rapid succession. Captain Abney first moistens the sheet of paper to be coated, and lays this on a slab of plate glass, after which contact with the glass is established by smoothing down the damp paper by means of a squeegee ; that is to say, a strip of soft india-rubber sandwiched between two laths of wood, so that about a quarter of an inch in width of the rubber strip projects. The slab of glass bearing the moist paper being now placed in a level position, a convenient quantity of the emulsion is poured upon it, and evenly distributed by means of a glass roller made as follows : — A glass tube, A, about an inch iu diameter and of suit ¬ sponge; but if a quantity is to be damped, a con venient way is to rapidly draw a bundle of a dozen sheets or so through water, to lay this bundle on a sheet of glass, after which a similar quantity of dry paper is laid on the wet bundle. Wet and dry bundles are now piled up alternately, and a heavy slab of stout glass is laid on the top, when the whole must be allowed to remain at rest until the moisture has diffused uniformly through the pile. It is scarcely necessary to say that the methods of coating just described are quite as applicable to the preparation of carbon tissue or of photo-lithographic transfer paper as to the coating of paper with emulsion. able length, and of which each end is closed by a cork (C C), forms the body of the apparatus ; and for conveni ence in use, a stout wire (B B) is run through the corks, as shown. In order to prevent the glass roller coming into actual contact with the paper, two rubber lings (DD) are placed round it, the exact position of these rings being determined by the width of the sheet of paper to be coated. It is convenient that they should be placed so as to be about a quarter of an inch from the edge of the sheet of paper when the roller is in use. It need scarcely be vemarked that the roller will drive any excess of emul sion over the edge of the glass plate, and a dish should be so placed as to catch this overflow. When the layer of emulsion on the paper has set, the sheet may be lifted from the glass and hung up to dry. Mr. Cowan’s arrangement for coating paper consists of a dish, A, containing the emulsion, and near one end of this is placed a glass tube, C, which may either be fixed, or may turn on a pin, like Abney’s roller. At about the same height as the top of the roller is a slab of stout glass, B, this being either supported by a board placed over the top of the dish, or in any other way which may be found convenient. A sheet of paper is now floated on the surface of the emulsion in the dish, and is next drawn over the tube C on to the slab B, as indicated in the diagram, E D representing the paper. As soon as the gelatine has set the sheet may be removed from the glass slab and hung up to dry. Emulsion for paper should not contain quite so much water as when used for coating glass, and it is very easy to remove any required proportion of water from the washed emulsion by soaking it for a longer or shorter time in methylated alcohol. It may also be mentioned that it is a very great advantage to previously prepare the paper with an enamel substratum, such as that recom mended on p. 619 of our volume for 1884. When the paper is coated by Cowan’s method or by any analogous process, the question whether it should be slightly damped first suggests itself, but in this case the worker must be influenced by the quality of the paper itself. If the sheets, when laid on the warm gelatinous solution, absorb moisture unequally, and, as a conse quence of this, become so much puckered and dis torted that they will not lie flat on the glass slab, there is no alternative but to damp the paper before commencing ; but if, on the other baud, the paper expands equally whilefloating on the emulsion, there is'no necessity whatever to moisten it. Single sheets may be moistened by means of a damp THE FUNCTION OF THE CONDENSER IN THE OPTICAL LANTERN OR THE ENLARGING APPARATUS. That it is possible to make a satisfactory enlargement by artificial light without using a condenser has been abun dantly demonstrated ; but this can only be done when the material on which the enlargement is to be made is so sensitive as to render a considerable loss of light a matter of no very great importance, but in other cases, the con denser is essential; aud indeed, one may consider the condenser necessary iu almost all cases where a photo graphic transparency is to be exhibited by means of the optical lantern. It is, however, a remarkable circumstance that the optical lantern is seldom so constructed as to enable one to get the full advantage of the condensing lens, and many users of the lantern have by no means a clear idea as to the relative positions which should be occupied by the light, the condenser, aud the objective in order to obtain the maximum of illumination and definition. Let us assume that the source of light is a point—a con dition of things impossible in practice ; but in the case of the limelight the radiant is small, and one may consider the matter as if the source of light were actually a point. The subjoined diagram shows what should be the relative position of the source of light, the condenser, and the objective ; the essential for good illumination being that the image of the source of light, as formed by the con denser, should be produced exactly at the optical centre of the objective, If this matter is attended to, not only is there a minimum loss of light, but one realises all the sharpness which would result from working with a small diaphragm; in fact, a diaphragm may be introduced into the lens without in any way altering the conditions. The most perfect lantern, whether for enlarging or for pro jection, is one in which light, condenser, and objec tive are immovably fixed according to the positions indi cated by the diagram, and the focussing is effected by shifting the position of the slide in the cone of rays. Such a lantern was, we believe, manufactured by Mr. George Smith, but the public preferred the ordinary model. If it is required to shift the objective P inwards towards the condenser, the balance of the system is overturned, and can only be restored by removing the light further from the condenser; while a movement of the objective out wards would make it necessary to place the light nearer in towards the condensing lens. If one had only occasion to use one objective on the lantern, it would be possible to so gear the lime-light holder and the objective together that both movements could be made simultaneously ; but there is often occasion to change the objective for one of another focus. The following diagram will illustrate the condition of