Having reached the practical limits in the reduction of the focal length of microscope objectives the lens makers turned to the prob lem of increasing the 'numerical aperture"— that is, an arbitrary numeral representing the product of one-half of the angular aperture of the objective by the refractive index of the medium between the cover glass of the slide and the outer lens of the objective. A high degree of success has been attained, so that dry lenses of 0.9, water immersion lenses of 1.2 and oil immersion lenses of 1.4 are avail able (against theoretical calculations respec tively of 1.0, 1.4 and 1.53).
To properly adjust the optical parts which have a fixed relation to one another by means of a tube (body) and to hold the object in its proper position in relation to them involves certain mechanical appliances which provide stability, convenience of adjustment and illumi nation: a rack and pinion provides coarse adjustment and a micrometer screw fine ad justment; both are extremely delicate; the stage or platform for placing the object, and mirror beneath the stage for reflecting abun dant light and a base for stability. This aggre gate of mechanical parts is called the 'stand.' The collar at the end of the tube to which the object glass is applied is the nose-piece; double and triple nose-pieces are also made to take two and three objectives, which may be rotated and focused on the object in turn. A mechanical stage provides delicate means of adjusting the object in place of the hands. The high powers require more than the usual amount of light for illumination and a con denser gathers it from the mirror .and con centrates it upon the object. Micrometers are provided to determine the amount of mag nification and measure the actual size of an object. A camera lucida attachment is made to project the magnified image upon a sheet of paper on the table to facilitate its drawing. Beside the microscope with single tube, thtre is another in which the rays from the objective are bisected and diverted into a second tube, so that the object may be viewed with both eyes. This is the binocular microscope. A special form of microscope known as the "micro-metallograph" is used for the examina tion of cold metals. The stage is above, with the objective below it and looking upward.
The eye-piece is at the end of a horizontal tube, the light rays being carried into it by means of a prism. Another prism on a lower plane is used to illuminate the lower side of the object. The so-called 'thermal microscopes is so modified from the usual model as to have a water-jacket around the objective to keep it cool while examinations are being made of heated objects in the small electric furnace provided as an accessory with the metallur gist's microscope. So far as our knowledge of the action of light and the constitution of visual images goes, the best microscopes now made realize about the limit to which the seeing powers of the instrument can be brought, al though experiments with especially controlled illumination, now in progress, may result in advancement in this direction. Nevertheless the field of research and discovery into which the microscope has as yet not penetrated, and which are within its powers, are almost unlimited.
The microscope is used as a necessary, acces sory in a large number of the sciences and in many industries. It is, primarily, the assistant of the teacher of biology, botany, bacteriology, histology, pathology and the allied branches of science. The medical profession employs it in the examination of the urine, blood and of can cerous and tumorous growths, as well as in searching for the parasites of the body, fungi which infest the hair and skin and for diagnos ing febrile diseases. For the examination of steel, iron and other metals to determine their intimate structure, the microscope is extensively used. Its use for the detection of adulterations in foods, drugs, paints, earths, starches and many other substances is often the only effec tive method of working. The Bureau of Ani mal Industry of the United States government depends wholly on the microscope for the detec tion of living parasites in the flesh of animals slaughtered for food. The microscope is used in many industries for counting fine lines, threads and fibres, and for determining the physical structure of cements, emulsions and other substances. The finest possible measure ments of space are made with the micro scope to which a filar micrometer is applied, and with it the rate of growth in plants is determined.