The Microscope in Metallurgy, Engineering and Spec troscopy.—In metallurgical work an inverted type of microscope is now usually employed, though an ordinary microscope fitted with apparatus for giving illumination from above can also be used. In the more commonly used form, the stage, the microscope proper, the illuminating apparatus and all the other equipment are mounted on pillars carried by slides which can be moved inde pendently along a machined bar, thus forming a type of optical bench. The specimen is illuminated from beneath, vertical or side illumination being provided by various types of apparatus. The illuminated underface of the specimen can be examined visually or can be photographed. The camera is an integral part of the apparatus and is mounted separately from the tube which carries the eye-piece used for visual examination. The same object-glass is used both for visual and photographic work, the change-over being made easily and quickly. Focussing adjust ments are fitted to the object-carrier and also to the object-glass support. The fine adjustment can be fitted to either, but is usually on the object-glass support in order that the load it has to carry shall be as small as possible.
The lenses used for metallurgical work should be selected with special regard to the absence of flare when used with a vertical illuminator. Special lenses of high numerical aperture (i.6) for use with (blue) monochromatic light have recently been designed specially for metallurgical purposes, to be as free from flare as possible, to be well-corrected for spherical aberrations and to have as high a resolving power as appears to be feasible with a lens computed for both visual and photographic use.
The mechanical properties of metals and alloys, such as hard ness, ductility and tenacity can largely be correlated with their structure as revealed by the microscope. The information pro vided by the microscope amplifies and extends that obtained from chemical analyses and in other ways, and enables the behaviour of the individual constituents to be more completely followed out when the metal or alloy is subjected to various heat-treatments or to mechanical processes such as forging, hot- or cold-rolling, drawing, pressing, etc.
Microscopes have valuable applications in the workshops as precision measuring devices. The type of microscope for use in the workshop is usually a simple robust instrument having a mag nification not greater than about X 20, with a low-power object glass and an eye-piece in which a scale or graticule is mounted. The type of graticule or scale used depends on the kind of work in connection with which the microscope is being used, as does also the method of mounting the microscope. For use on flat surfaces the microscope may be rigidly mounted on a plane base, while for other uses, such as in precision screw-cutting, tool- and jig-making, etc., other types of support are used. The microscope is provided
with sliding-tube focussing, or with an ordinary rack-and-pinion coarse adjustment. Simple electrical illuminating devices are fre quently incorporated with or permanently fitted to these instru ments. For examining and measuring screw threads, wires, etc., specially designed microscopes, some of which are of the "projec tion" type, are used.
It is only possible briefly to point out the properties desirable in the type of attachment which enables the microscope to be used for micro-spectroscopy. It should be so constructed that the object can be brought into the centre of the field and isolated by means of suitable adjustable diaphragms, and arrangements should be made for the spectroscope and slit to be swung in easily when the object and illumination have been properly adjusted. Pro vision should also be made for comparing the absorption spectra seen in the microscope with a wave-length scale in the eye-piece and with spectra obtained from known substances. The micro spectroscope has many useful applications in the study of colour ing matters of vegetable, animal and mineral origin, and can play an important part in micro-chemical work and in the identification of small quantities of fluids or solids having selective absorption for light.
The manifold applications mentioned or indicated in this article show that the microscope can be used for studying by means of light, anything which, as regards size and condition, can be pre sented to the instrument. It has been said earlier that there is no need to stress the importance of the microscope in medical, biolog ical, geological, metallurgical and many other applications, but it is well to call attention again to its importance in general scientific and industrial work and research. An hour or two at the micro scope may not only assist in shaping the problem to be investi gated, but it may in many instances enable much laborious work to be avoided, either by solving the problem completely or by indicat ing those methods of attack most likely to bring a speedy solution. If care be taken to become familiar with the general principles and methods of microscopy, a comparatively small additional experi ence in any one particular line of work will make the microscope practically indispensable for such work. The many valuable appli cations of the fnicroscope in science, industry and general educa tion have rightly over-shadowed its use as a means of recreation alone ; but it is well to recall the interest, pleasure, enlightenment and inspiration which the microscope can arouse, when used for recreation, as a revealer of unsuspected beauties and of things otherwise unknowable.
For bibliography see MICROSCOPE. (H. JA. ; H. Mo.)