William C. Roberts-Austen and Francis H. Neville

“ Tinted crystals may thus be distinguished from the investing layer caused by the presence of a minute quantity of another constituent. The temper colours produced by heating iron or steel in air are well known. Carbide of iron is less oxidizable than the iron with which it is intimately associated, and it assumes a brown tint, while the iron has reached the blue stage. ”

“ Every micrographer has his own views as to the form of an installation to be adopted, and it will therefore be well to give an illustration of a definite apparatus which has been found to give satisfactory results. It consists of a microscope A with a firm base placed in a horizontal position. The microscope can be connected by a tube B with the expanded camera CC, at the end of which is the usual frame to receive the photographic plate. A practised observer can focus on a plate of clear glass by the aid of a subsidiary low-power microscope lens. ”

“ In some cases the alloy may be attacked electrolytically by exposing it for a few minutes to a weak electric current in a bath of very dilute sulphuric acid. Certain organic bodies give very satisfactory results. The Japanese, for instance, produce most remarkable effects by simple reagents of which an infusion of certain forms of grass is a not unimportant constituent. ”

“ Tincture of iodine also affords a means of identifying lead in certain alloys by the formation of a yellow iodide of lead, while the vapour of iodine has in certain cases been used to tint the constituents. Thin coloured films may often be produced by the oxidation of the specimen when heated in air. ”

“ Polishing in Bas-Relief.—If the polishing be completed with fine rouge on a sheet of wet parchment, placed upon a comparatively soft base such as a piece of deal, certain soft constituents of an alloy may often be eroded in such a manner as to leave the hardest portions in relief. ”

“ It is necessary that the lens used for concentrating the light on the illuminator should be an achromatic one, as colour effects cause trouble in photographing the objects. For lower powers the Lieberkuhn parabolic illuminator is useful. Certain groups of alloys show better under oblique illumination, which may be effected by the aid of a good condensing lens, the angle of incidence being limited by the distance of the object from the objective in the case of high magnification. ”

“ A section through the culot so obtained will show a gradation in structure from pure metal on one side to pure metal on the other. A thin slice of metal is usually cut by means of a hack-saw driven by mechanism. ”