Volltext Seite (XML)
56 THE PHOTOGRAPHIC NEWS. [May 28, 1869. bruising or grinding, rather than by striking ; those sub stances which are softened by the heat, occasioned by rubbing, and percussion, require’to be pounded very slowly ; those which are hard must first be pounded by repeated strokes of the pestle until well bruised, and then powdered by grind ing. Care should be taken, in pounding in a wedgwood, marble, agate, or glass mortar, that the pestle does not strike the side of it, otherwise it will most likely be broken. In powdering precious substances, great loss may arise, unless care is taken, from the fine powder which, from some substances, arises abundantly during the operation, and great inconvenience incurred from the same cause when powdering poisonous or irritating substances, such as corro sive sublimate, cantharides, and some others. In these cases, a cloth or skin should be so arranged over the month of the mortar as to prevent the powder from escaping, and yet allow the free action of the pestle. In cases where it will not do injury, a few drops of water may be added. This caution should be particularly borne in mind, otherwise the health of the operator may be greatly injured, particularly should the quantity be large. Bruising or coarsely powder ing a substance always aids its solution ; this, in chemistry, signifies an intimate union of solid with fluid bodies, so as to form a transparent liquid. Simple solution implies a suspension or dissolving of the substance in the fluid, from which it can be recovered in its previous state by evapora tion ; as, for instance, nitrate of silver dissolved in water. In chemical solution, the nature and properties of the substance become altered, and it cannot be recovered in its pristine form except by chemical decomposition ; as, for instance, metallic silver dissolved in nitric acid. No photo chemical manipulation varies mote than this, and the success of very many processes depend upon an accurate knowledge of the proper solvents, the way of applying them, and the degrees of solubility, in such solvent, of the solid under examination. In some cases heat will be re quired to aid and hasten the solution, as in dissolving gums and resins for making varnishes; in others, frequent shaking or stirring may be needed. Bruised glass is often used to prevent gums from coagulating in a lump, and so preventing the action of the solvent upon them. Great care should be exercised in dissolving substances in volatile and inflam mable liquids by heat. Many sad accidents have happened in this way, from either want of knowledge or carelessness. Suppose a composition of shellac, bee’s-wax, and turpentine or benzole, is required ; the boiling points of these substances vary so considerably that, should they be placed together in an open vessel over a fire, the turpentine or benzole would, in all probability, be evaporated, or on fire, before the shellac had melted ; therefore it would be best to proceed as fol lows :—Procure a ladle, or iron pot, big enough to hold double the quantity of the mixture required ; first put in the shellac ; when melted, put in the bee’s-wax, and stir until melted ; take the pot or ladle off the fire, and gradually stir in the turpentine or benzole, which has. in the meantime, been warmed by placing the bottle which contained it in hot water, leaving it uncorked. Should the mixture need further diluting, more turpentine or benzole can be added when cold ; but the mixture should never be replaced upon the fire in an open vessel, otherwise an accident may occur through its bursting into flame. It is well to have at hand a wet cloth, and either an old plate, a piece of slate, or iron, which will lie flat and completely cover the ladle. This placed over it will damp down the flame and put it out without danger. Never attempt to put out the flame of a burning liquid by blowing it with the breath, or you will run the imminent risk of scattering it about the place or over your clothes, thus damaging the furniture and risking a loss of life itself. The composition above forms, with colour added, common sealing-wax, or, with a greater proportion of turpentine or benzole, a good waterproof varnish for wooden tanks or dishes for photographic purposes. 114, High Street, Ilfracomhe. THE PHOTOGRAPHIC PLANE-TABLE. ^Extract of Report of MM. Rognault, Fizeau, and Abhadie, to the Academic des Sciences, upon the Photographic Plane-table invented by M. A. Chevalier. When Daguerre first employed the camera for the purpose of fixing images upon a metal plate, Arago at once pointed out the fact that topography might borrow from photo graphy its rapid means of registration. The view taken by this great physicist gave rise to divers projects to bring about a solution of the problem, and five years later, M. Martens showed how it was possible to fix upon the surface of a cylinder a succession of images received through a narrow slit by means of a moveable lens. In this manner half the tour of the horizon only was obtained ; but M. Garella, Engineer-in-Chief of Mines, proceeded to perfect the instrument in such a way as to render it possible that the images might be taken upon a plane surface, and that the whole tour of the horizon might be secured. The details of the instrument were exceedingly ingenious; but when it was desired to construct a plan, long calculations and deductions were necessary, causing delay and giving rise to the possibility of errors. Seventeen years after the invention of Daguerre, A. Chevalier stated, in very precise terms, the problem to ba solved, establishing the fact that in order to obtain a plan upon paper, it was necessary to construct, at each station, a special protractor, formed photographically by the whole of the surveying signals, which the instrument registers automatically, according to the angular separations as observed by the eye from the station selected. This result M. Chevalier obtained by causing a vertical lens to make the tour of the horizon, by means of clock work mechanism, and thus to secure images of the signals situated around the station. These images are thrown by means of a reflecting prism, in rigid connection with the lens, through a slit placed above a horizontal sensitive plate. The axis of this slit forms part of the moveable vertical plane, which contains at once the optical axis of the lens and the centre around which it moves in azimuth. The slit opens automatically, and without vibration, after the lens and the prism have already acquired a uniform rotatory movement. When the tour of the horizon is finished, the slit closes of itself, and the whole apparatus may be turned horizontally until the needle of a compass attached coincides with the zero upon its disc ; then, by opening an aperture specially constructed for the purpose, the light is allowed to trace on the margin of the plate a line indicating on the picture the direction of the magnetic meridian. This pre caution serves to give the direction to the negative, and allows of the draughtsman, when constructing his plan, to identify the several combinations of subsequent tours of the horizon. In this circle of photographed images, all the signals pre serve between themselves the true angular separation as seen from the station. In rare cases, where a signal is too high or too low in relation to the instrument, a small lens d eclimitre, moveable round a horizontal arm fixed upon the apparatus, allows of recording this signal upon a landmark in the field of the instrument. Finally, a hair line, pro duced by means of a thread situated at the height of the optical axis, shows all the points in the photographic pano rama, in which the height is equal to that of the station whence one operates. The employment of the apparatus requires no other special knowledge than that of photo- graphy. . , , . To comprehend the great simplification thus brought about in the art of surveying, let us repeat the details of tho methods hitherto used :— After having, with considerable trouble, directed the optical axis of the telescope of tho theodolite separately upon each signal, the figures of the levels are taken down in succession. When the levels are very numerous, several hours are necessary for making one entire tour of the hori zon. A protractor is now placed in position, and, after