Page:The New International Encyclopædia 1st ed. v. 13.djvu/505

MICROSCOPE. 455 MICKOSCOPICAL SOCIETY. to use the product of the sine of half this angle by the index of refraction, as indicating the ef- fective aperture irrespective of the method of using the objective, and this constant is called the numerical aperture. The resolving power of an objective must not be confused with the mag- nifying power, for theoretically any desired de- gree of magnification can be obtained, but there is a detlnite limit to the resolving power set by dillraction phenomena, as pointed out by Dr. Ablx. Owing to the fact that a lens on account of diU'raetion is not able to form an actual point, as the image of a point, it is evident that if the little rings which are formed overlap, then no degree of further magnification can separate them, and they will confuse the vision. It has been shown that the success of an objective in gather- ing in all the components due to diffraction is directly dependent upon the numerical aperture. Abbe has calculated that the theoretical limit of resolving power for an aperture of 180° would be lines about 120,000 to the inch, falling to about 95,000 for 107°. This has been nearly reached in some of the best instruments. The- oretically two lines must be distant from each other at least X /2a, in order to be seen dis- tinctly, where a is the numerical aperture and is the wave length of the light. In order to make use of the highest efficiency of the objective it is necessary to devote niucli attention to the concentration of the light upon the object in order that the image may be well lighted and also that the full aperture of the objective may be utilized. A form of condenser which is placed under the object is shown in the accompanying plate (Fig. 4) ; Sp is the mirror for reflecting the light into the condenser >S, and the rest is mechanism for suitable adjustments. The adjoining figures show the section of such a condensing lens. 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,' D, enabling the observer rajiidly 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 possilde 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 tJie focus of a lijgh- 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 'polarizer.' 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 polarizer or the condenser may be 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 ujwn 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 jwlarized light and identify their optical properties and determine completely their nature. The petrographic miscroscope 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 Stereo-scope. ) In a binocular microscope there is the usual arran^e- 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 u.sed 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 comes 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 cases, and by that means photographs may be taken of the objects under investigation. For the early history of the microscope, the reader should consult Ger- land and Traumiiller, Geschichte der Experi- mentierkunsf (Leipzig, 1899). Drude. Lchrbuch tier Opiik (Leipzig. 1900). translated into Eng- lish as The Theory of Optics bv Mann and Jlili- 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 Micro- scopy, Cltnical. MICROSCOPICAL SOCIETY, The Ameri- c.A,N. An assoeiatiim organized in 1878 and in- corporated in 1891, at Washington, D. C. It has