versed negatives for collotype and other purposes, it is a good plan, when possible, to have an arrangement for laying the plan horizontally under the lens at any convenient distance from it. The whole of the apparatus connected with the camera and plan-board must be rigid and firmly fixed, so as to be free from vibration. The slightest vibration is sufficient to destroy the perfect sharpness of the image. In the glass-house attached to the Photographic Branch of the Surveyor General's Office here, I have endeavoured, and I think with success, to overcome all vibration caused by carriages passing in the street close by, by dividing the floor of the camera-room into isolated blocks resting on a bed of sand, so that each camera shall stand by itself on a block isolated from adjoining blocks and from the walls and floor of the building. The planboards are fixed on a separate wall quite isolated from the walls of the building. Plans may be copied either in the open air or under shelter-coloured and old stained manuscripts, maps or drawings are better copied in full sunlight. The glass-house I have constructed at the Surveyor General's Office faces the south and is glazed with ground glass, so that a strong diffused light may be thrown upon the plan-boards. When circumstances. permit, it is well to have the camera and plan-board mounted on a firm stand working on a pivot, so that, as the day wears on, the position of the plan-boards may be changed so as always to face the sun. I adopted this arrangement at the Trigonometrical Survey Office, Dehra Dun, and I believe it has many advantages over the fixed glass-house rendered necessary in Calcutta by the constant wind and dust, and the greater necessity of being able to carry on work without interruption at all times of the year. The negatives of maps &c., drawn in line only, for reproduction by photozincography, are taken by the ordinary wet collodion process with iron development, modified so as to secure the greatest transparence in the lines. and density of the ground; but as the ordinary wet collodion process by itself will not give all the intensity required to produce an almost opaque ground, it is obtained by intensifying the negative in the usual way with pyrogallic acid and silver, after fixing; then treating it with a saturated solution of bichloride of mercury till the film becomes white, and finally applying a dilute solution of hydrosulphate of ammonia, which instantly changes the colour of the film to a dense black or brown throughout. The negative is afterwards varnished with a resinous varnish, or flowed over, while wet, with a solution of gum or gelatine and allowed to dry. All defects, pinmarks &c., are then stopped out with Indian ink or black varnish. In taking the large negatives on plates 32 × 24, that we are now producing for copying the maps of the Cadastral Surveys, it has been found that the first intensification may be produced by washing the plate after the first develop ment and applying a weak solution of nitrate of silver followed by a second application of the iron developer. Other methods of obtaining the extra density required for these negatives have been proposed and are in use,* but, notwithstanding several inconveniences arising from the use of bichloride of mercury and hydrosulphate of ammonia, the above appears to be the best and most certain when working on the large scale. When maps are not drawn entirely in pen and ink but have the hills brush shaded, and it is desired to reproduce them by the collotype or engraving processes, great care and skill are required on the part of the photographer to get the ground of the negative dense enough to give a perfectly clean impression in the white parts of the map, and at the same time prevent the grain of the paper from showing and give the faintest tints of the shading their proper value. Coloured maps also give a good deal of trouble, and when allowable the colour should be washed off as much as possible before the negatives are taken. Colours may sometimes be removed by chemical means, but there is risk of injury to the original. use In many of the processes about to be described it is necessary to a negative which instead of giving an image reading the same way as the original shall give it reversed as to right and left. There are several methods of obtaining these reversed negatives. (1.) By coating the original unreversed negative with a thick transfer collodion,† or a layer of gelatine, and then stripping off the film and, either laying it down again on a sheet of glass in a reversed position, or using it as a film negative which may be used for giving both reversed or unreversed images. This method is practical and useful, but is not suitable for map work on account of the liability to contraction and distortion of the image. There is also difficulty in keeping the film negatives flat, and they have been found to become brittle and perish very soon in this climate. (2.) By turning the sensitive plate in the camera, so that the light acts, through the glass, on the back surface of the film. This method is also practical, simple and useful, but requires care in the selection of glass plates free from scratches, &c., and in wiping the back of the plate before it is put in the camera. It is used in the Belgian Topographical Bureau with dry tannin plates, but I have not found it suitable for reproducing very fine map-work on wet plates. (3.) By placing a reversing mirror or prism in front of the lens-the image thus passes through the lens reversed and is impressed directly on the sensitive plate. This is one of the simplest and most effectual of all methods. If a mirror is used it should be one silvered by depositing silver * See Abney, Instruction in Photography, p. 22. † See the same work, p. 160. on the front surface and should be large enough not to cut off the oblique rays entering the lens. A large solid reversing prism is expensive and heavy, but for moderate sizes an efficient instrument may be made by building up a hollow prism with glass plates and filling it with a transparent fluid having a suitable index of refraction. M. Derogy, of Paris, has just invented an ingenious and economical method of employing a reversing prism by placing a small prism between the lenses. I have not seen any results of this arrangement, but it seems likely to be as effective as it is simple. (4.) By what is known as the dusting on' or 'powder' process. A glass plate is coated with a mixture of gum, sugar and bichromate of potash dissolved in water, thoroughly dried with heat, and then exposed to light under a negative. After removal from the printing frame, the gummy film is dusted over with very fine plumbago which adheres to it in inverse proportion to the action of light, i. e., those parts on which the light has acted refuse the powder in proportion to the intensity of the action of light, while the protected parts, attracting moisture from the air and so becoming 'tacky,' take the powder readily, and thus an exact transcript of the original negative is produced, but reversed. This method is simple and effective and seems to be one of the best that can be employed when a mirror or prism is not available, or when, as is frequently the case, the reversed negative can only be obtained by copying from a single original unreversed negative. (5.) By making a copy, either in the camera or by contact, on a film of collodio-bromide of silver. The image is developed as usual by the alkaline method, and then treated with nitric acid which dissolves the reduced silver in the exposed parts of the film, leaving the bromide in the unexposed parts; the plate is then again exposed to light and developed. This method, proposed, I believe, by the late Mr. Sutton, is said to give very good results, but the use of nitric acid is an obvious disadvantage. (6.) By copying in the camera, first making a transmitted positive either by the collodion process or with a special pigmented gelatine tissue. This method is most useful when the reversed negative is required to be either larger or smaller than the original. (7.) By means of the reversing action of the red and blue rays of the spectrum. This method is a discovery of my own and has not yet been thoroughly worked out; it is, however, simple and could, I believe, be successfully utilised. A film of collodio-bromide of silver stained with anilin blue is exposed to light for a few moments, then placed under a negative in a printing frame in front of which is a sheet of red glass, and exposed to light. The action of the light passing through the red glass in the clear part of the negative is to neutralise or destroy the effect of the previous exposure of the plate to light, and on development a more or less perfect reversed negative image is obtained. For line subjects a blue glass may also be used, but for half-tone work only a red glass can be employed. In some processes also, it is convenient to use either direct or reversed transparent positives instead of negatives. These can be obtained either by contact printing on dry collodion plates or gelatine tissue, or in the camera, in the manner adopted for making transparencies, as described in the text-books. Having now described the preliminary operations for preparing the original and producing the negative, which are common to all processes, we may proceed to the consideration of the different printing processes which, as stated in the introduction, may be divided into 6 classes, viz. : I. Printing on sensitive papers. II. Photo-lithography or Photozincography. III. Photo-collotype. IV. Woodbury-type. V. Heliography or Photo-engraving. VI. Photo-typography. IV. PHOTOGRAPHIC PRINTING ON SENSITIVE PAPERS. The processes under this head may be divided into three classes- of silver and the results are not permanent. Secondly :-Those in which the sensitive papers are prepared with the salts of iron, platinum and other metals, and the prints though not absolutely permanent are more so than silver prints. Thirdly-Those in which coloured gelatine or other colloid mixed with an alkaline bichromate forms the sensitive surface and yields prints which, for all practical purposes, may be considered perfectly permanent. Silver-printing-Notwithstanding its expensiveness and the want of permanence of the prints, silver printing has hitherto maintained the first place among photographic printing processes, and though very nearly equalled, is as yet unsurpassed for the beauty and delicacy of its results. It is the process in most extensive use for producing copies of portraits. and views, and although rapid advances are being made in more permanent methods, it is likely to be a long time before the beautiful but perishable silver print is entirely superseded. The following brief outline of the operations will be sufficient to show the nature of the process.* A sheet of paper coated with albumen containing an alkaline chloride, such as common salt, or paper which has merely been immersed in a solution of such salt and dried, is floated on a solution of nitrate of silver and allowed to dry in the dark. It is then placed above the negative in a copying frame, which is so constructed that the light may pass freely through the negative, and at the same time may admit of the examination * For details, see Abney's Instruction in Photography, p. 113. of the print while the back surface of the sensitive paper is shielded from light. The exposure to light lasts for some minutes, by which the parts unprotected by the denser parts of the negative are darkened more or less, according to its translucency, while the parts entirely shielded from the light remain quite white. When the action of the light is judged sufficient, the sensitive paper is removed from the frame in a dark place, and must then undergo an operation of fixing to remove the unchanged salts of silver, which would cause the print to darken unless carefully protected from the light. This is effected by steeping the print for a short time in a solution of hyposulphite of soda; but before the print undergoes this indispensable operation it is usual to place it in a solution of chloride of gold, by which part of the reduced silver forming the image is replaced by a film of gold and the print takes a more agreeable tone, also becoming more permanent than it would be if this operation, called 'toning,' were omitted. It is, however, impossible to ensure perfect permanency of these prints, by reason of the sulphur contained in the albumen or in traces of sulphur salts formed by the decomposition of the hyposulphite of soda, and left in the print after even the most careful washing, slowly acting on the reduced silver forming the image and converting it into a sulphide, by which the tone and brilliancy of the picture are lost, and the lighter shades appear to fade away entirely. It should, however, be stated that prints prepared on plain, or unalbumenised, paper are more permanent than the albumenised prints, though not so brilliant, sharp and delicate; and they have the further advantage of being less liable to shrinkage and distortion than the albumenised prints, and are thus more suitable for the reproduction of maps where accuracy of scale is a desideratum. It is evident that owing to the expensiveness of the materials used in producing these prints and their want of permanency, together with the slow rate at which they can be produced, this process is almost useless for the reproduction of maps in large numbers; and, in fact, its use in cartography is limited to making copies of special maps for immediate reference or temporary purposes, and as guides for engravers or lithographers in preparing compilations from maps on a larger scale. Silver prints have also been used by engravers to obtain a correct tracing on the waxed surface of their copper plates, but unless these prints are prepared with great care they are open to the objection of becoming distorted and untrue to scale by the contractions and expansions caused by the successive washings they have to undergo. Prints on plain paper are better for this purpose than those on albumenised paper, and prints on paper containing a large proportion of resin in the size are better still. In the English Ordnance Survey the topographers are furnished with silver-print reductions from the large scale outline survey, on which |