On the accompanying plate (Fig. 2) is shown a modern microscope of a high order as fitted for general and biological work. The main stand S is so hinged that the top may be tilted at any angle and clamped by the lever M. The 'tube' A carries at its lower end a 'triple nose-piece,' enabling the observer rapidly and easily to exchange objectives, C, F, etc. In the upper end of A is the 'draw tube,' B, enabling the observer to change the distance between his objective, C, and eyepiece, E. LCKJ is the stage or table on which the objects are placed. K is a vernier reading the angular rotation of the stage. L and J are milled heads operating the mechanical stage. making it possible to move the object regularly up and down or right and left in searching for an object in the slide, in counting, and the like. I is the substage condenser and its mounting, including a diaphragm. NO is the rack and pinion for rough adjustment of the focus, and G is the fine adjustment making it easy to adjust accurately the focus of a high power objective and in some cases to make measurements.
A similar instrument as fitted for petrographic work where polarized light is used is also illus trated on the plate. This particular microscope is not fitted with a mechanical stage. At P is in troduced a 'pelarizer,' Nicol prism for furnishing a beam of polarized light, and another Nicol prism used as the analyzer is slid into the side of the tube at R or for other combinations at S. U is a rack and pinion for the adjustment of the draw tube, B. For some purposes the analyzer is put on top of the eyepiece at T. Either the polarize!' or the condenser may he turned out from under the stage when not wanted. Between the objective and the analyzer is a side slot, into which may be introduced the quartz wedge, mica plate, etc., which are used in the determination of the optical constants of the minerals under study. For use in such instruments the rock to be investigated is ground to a very thin sec tion mounted upon a glass strip, like any micro scope preparation. Under these circumstances most minerals are quite transparent and the stu dent is enabled not only to learn the size and form of the grains, but also to subject them to an investigation under polarized light and identify their optical properties and determine completely their nature. The petrographic miscroseope has revolutionized the study of rocks.
There is also the binocular microscope, in which two eyepieces are used in order to secure a stereoscopic effect. (See STEREOSCOPE.) In a binocular microscope there is the usual arrange ment of the objective, but one or more prisms of special design are interposed so as to deflect some of the rays to a second eyepiece. It is unavoid able that a certain amount of light is cut off in passing through the prism, or that the path of the rays is increased so that where the high est powers are employed the binocular is not used.
There are used in connection with the micro scope many forms of the 'camera lucida' (q.v.), a device to enable the operator to make a drawing of the object under study by tracing over the virtual image which he seems to see on the paper as the eye of the observer sees both the light which conies up from the object and that which comes from the paper and pencil be low the miscroscope. In preparing slides for work in microbiology it is necessary that the ma terial should be in very thin sections, and this is accomplished by imbedding the whole object in paraffin and then shaving off' thin sections with a `microtome,' one form of which is shown on the accompanying plate (Fig. 8). After wards the paraffin is removed, and the shaving mounted upon a glass slide. These preparations are usually hardened by chemicals, and are fre quently dyed with special solutions, which may. for example, color the nerves and not the other parts, thus bringing out the contrast, and as sisting the work. Under some circumstances the object is frozen by means of liquid carbonic acid gas and then shaved in sections.
It should be evident that by a simple device a camera may be made to take the place of the eye in any of the above eases, and by that means photographs may be taken of the objects under investigation. For the early history of the microscope, the reader should et:insult Ger land and Traumfiller, acschiehte der Experi menticrkunst (Leipzig, 1899). Drude. Lehrbuch der Opiik, (Leipzig, 1966). translated into Eng lish as The Theory of Optics by Mann and Mili kan (New York, 1902), should be read for the theoretical side. A practical and complete trea tise is Carpenter. The Microscope (8th ed., edited by Dallmeyer, Philadelphia, 1901 ). See Mica°. SCOPY, CLINICAL.