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MECHANICAL

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Originally appearing in Volume V18, Page 406 of the 1911 Encyclopedia Britannica.
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MECHANICAL ARRANGEMENTS Although the See also:

optical. See also:system is the first See also:consideration in a See also:microscope, the system is valueless if the fittings do not allow its correct use. The optical system must be kept at a certain distance and well centred, and a correct position for the See also:object in relation to the system must be assured. In fig. 6o, See also:Plate, the microscope is seen to consist of the heavy See also:metal See also:foot A, which rests on the table at three points. The whole microscope is fitted to this foot. The object can be held firmly on the See also:stage plate B by cramps C. On the See also:lower See also:side of the stage plate are the See also:condenser and the diaphragms, and the See also:illuminating See also:mirror J is held by a See also:rod D fixed to the stage plate. Likewise on the stage plate is the support for the See also:tube E. The rough See also:adjustment of the microscope can be made by a See also:rack and pinion F; and the See also:fine adjustment by the See also:screw G. The tube containing the eyepiece and the See also:objective is See also:double. The inner tube H is movable, making a See also:change in the length of the tube possible. As a See also:rule this inner tube has a See also:mark which allows the length of the tube to be set.

It is most important the stand should be See also:

free of vibration. A fine adjustment is also necessary, in See also:order to perform conveniently and with certainty the slight See also:motion of the microscope in relation to the object. In cheap stands the rough adjustment was worked by moving the inner tube by See also:hand, but the more convenient rack and pinion is now used almost exclusively. For slight magnifications rough adjustment is sufficient, but with objectives of a See also:focus below i in., a fine adjustment is wanted. Very different constructions are in use. Almost all are such that the whole microscope tube is raised or sunk by the mechanism of the fine adjustment, and not only the objective. The most used is the See also:micrometer screw adjustment (fig. 51). The tube See also:carrier B T -T J.SWIFI. SON See also:LONDON A B c• Er fits closely on to a See also:column A which is fixed firmly to the stage plate The end of the column C is traversed by the micrometer screw I] which is set in See also:action by the knob E. The column A contains a powerful See also:spiral See also:spring, which exercises a strong pressure on the plate F fixed to the carrier B. By screwing in the micrometer, the spring is compressed and the tube lowered.

By the contrary See also:

movement the spring pressure raises the tube as far as is allowed by the screw. The strong pressure of the spring practically excludes motion, which with fine adjustments is very important. Another very See also:good adjustment is that of Messrs See also:Swift & Son, shown in fig. 52. The See also:long See also:lever D is pressed to one side by the screw F, and is thus turned See also:round the See also:pin E. On the tube very near to the pin E is a See also:cylinder C, which by the action of the screw F is very slightly raised or lowered. A double lever is used in a fine adjustment by Messrs See also:Watson & Sons (fig. 53). According to whether the screw A or B is used, the adjustment is fine or coarse. In other fine adjustments by means of springs and See also:balance wheels either a micrometer screw is moved (Zeiss), or a curved disk fixed to the balance See also:wheel is turned (Leitz), or an oblique disk arranged more or less in a circle and attached to the balance wheel is revolved (Reichert). These See also:modern adjustments are made so exact that motions can be easily measured Prisms. a.

Wenham's See also:

Prism. See also:Powell's Prisms. up to o•oo2 mm. An essential in all rough and fine adjustments is that the motion must always be parallel to the optical See also:axis of the microscope, so that the same point in the object remains in the centre of the See also:field. Another See also:condition which must be fulfilled by a good stand is the See also:power of inclination. It is only rarely necessary to arrange the preparation really See also:horizontal; and for easy observation, especially when it will take a long See also:time,.it is of See also:great assistance if the micro-See also:scope can be inclined, so that the observations can be made in a natural position. The apparatus for inclining the microscope is chiefly such that the micro-scope can be placed in all positions between the See also:vertical and the horizontal. The horizontal position is sometimes necessary if photographs are to be taken by the microscope. Many devices are available for changing the objective. It is essential .that the objective is always brought before the lower end of the tube in such a way that the optic axis of the objective coincides with the optic axis of the See also:rest of the system. The fittings of the objective and the changer are so arranged that little or no fine adjustment is necessary after the change. The most widely used is the revolving changer (fig.

6o, Plate). The revolver may hold two, three or four objectives. In the sliding changer the objective is, dovetailed to a slide, the correct position being secured by clamps. Fully equipped microscopes have apparatus for moving and turning the object. In See also:

simple microscopes the stage plate lies on the stand held by two springs, and must be moved by the hand (fig. 6o, Plate). For elaborate See also:work a so-called See also:cross-table is indispensable. By means of screws the stage plate is movable in two directions at right angles to one another, in the See also:plane of the stand. In many cases the stand is also movable round the optic axis. The microscope stands described above can be used for the greater number of the naturalist's experiments. For very See also:special See also:objects the stand must be expressly made; thus stands with tube See also:carriers very much projecting are made for examining sections of the See also:brain. The petrographical microscope is shown in fig.

61, Plate. In order to determine the refractive See also:

index when the thickness of the crystal is known, or the thickness of the crystal when the index is known, a fine adjustment A makes it possible to measure exactly the changes in the length of the microscope. Further, a revolving stage plate provided with a See also:graduation B is used to determine the See also:angle in crystals. To obviate mistakes the optical axis of the micro-scope must coincide with the revolving axis of the plate, and the revolving plate has a central position C to keep this condition fulfilled. In many stands the objective can be centred instead of the plate. For measuring this angle, an eyepiece with cross-threads is used. In the lower See also:focal plane of the eyepiece, at the spot where the real See also:image which the objective forms of the object arises, a See also:glass plate is introduced on which are two fine cross lines or even two very thin threads. The See also:eye-See also:lens can be adjusted for the See also:thread-plate, so that different observers can see the cross clearly. The cross is always adjusted first. When observing with such an eyepiece, care must be taken that the real image of the object lies in the plane of the cross-threads, i.e. that there is no See also:parallax. The adjustment is easily controlled. If the eye is moved to and fro over the eyepiece and the image makes apparently similar movements in relation to the cross threads, then the image does not yet See also:lie in the plane of the threads.

To measure the angle, the images of the crystal edges are covered in turn by one of the threads by turning the table, and the angle of rotation is read from the See also:

scale. A cross-table is very convenient for this calculation, for with the aid of the two movable slides situated in the plane of the plate and at right angles to one another, the point where the two crystal edges intersect can be quickly and correctly brought into the revolving axis of the plate. This measurement can also be made with a See also:goniometer eyepiece, in which a See also:row of parallel double-marks are used instead of the cross threads. The fitting of the eyepiece at the upper end of the tube is provided with a graduated circle. The eyepiece proper with the parallel strokes can be revolved, and the rotation be read from the graduated circle. In carrying out this calculation the marks of the thread-plate have only to be placed exactly parallel tc the crystal edge. For examining preparations in polarized See also:light a polarizer D is introduced in the illuminating apparatus below the See also:diaphragm and an analyser E above the eyepiece. The analyser can be rotated, the angle being read by a divided circle F. Very often the analyser is placed in the tube, a little above the objective: it is then generally in a See also:case G, which can be put into the tube. The placing of the analyser near the objective has the See also:advantage that the field of viewsize of objects or parts of objects. There are three essential ways of performing this. The first method uses the objective screw micrometer.

The object is placed on a slide in the plane of the stage plate and able to be very finely moved by the micrometer screw, which has as fine a See also:

worm as possible. A divided cylinder is fixed to the turning knob, which thus makes it possible to measure fractions of the revolution. The revolutions of the cylinder are registered by a calculator. The use of an eyepiece with a cross thread is essential to this measurement. After the microscope has been so adjusted that the image of the object to be measured falls exactly in the plane of the cross threads, the object is moved by the micrometer until one edge of the object is exactly covered by a thread. The micrometer is now read. Then the object is moved by the micrometer till the image of the other edge is covered by the thread in the eyepiece, and the micrometer is again read. The difference between the two positions gives the See also:size of the object. The objective screw micrometer is, however, not sufficiently delicate, and is only used when comparatively large objects are to be measured, and especially for objects whose edges do not appear at the same time in the field of view. The second and most widely used method employs a micrometer eyepiece. In this case not the object itself but a real image which has already been magnified by the objective is measured, and obviously much more accurate results are possible. The most accurate calculations are obtained by using the screw micrometer ocular (fig.

54). Directly below the collective lens of a See also:

Ramsden eyepiece a slide b can be moved by a micrometer screw a; the slide carries a little glass plate c provided with a graduation. With the help of this scale the See also:total revolutions of the screw can be read; fractions of the revolution can be read from the divided cylinder d. The scale is generally divided into hundredths of millimetres or thousandths of inches. A fixed mark which serves as an index is placed on the lower side of the collective lens and is seen clearly at the same time as the graduation of the movable slide. The micrometer stands at zero if the zero mark of the cylinder coincides with the index and the fixed mark is at a known See also:division. The calculation is most convenient if the micrometer is See also:left in the position of zero and the object is moved till one of its edges corresponds to the zero mark of the eyepiece scale. If the micrometer is then moved till another graduation corresponds to the other edge of the image the size of the image can be read off. As this method See also:measures ~DnDD ment of Watson & Sons. is not restricted, as is the case if the analyser is used above the eyepiece. Nicols's, Glan-See also:Thomson prisms or similar polarization apparatus are used as polarizers and analysers. Below the analyser G a plate H of selenite or See also:mica may be put in the course of the rays.

This small plate can also be laid above the polarizer in the illuminating apparatus or in the eyepiece. To examine crystals, especially in converging light, a condenser, movable in the optic axis, is needed above the polarizer. The image produced by the microscope objective M in its back focus plane is then observed through a supplementary microscope. The objective of this supplementary microscope, the See also:

Bertrand lens, can be applied through a window I at the lower end of the inner tube K. By using a rack and pinion movement L the supplementary microscope can be adjusted for the images. There is nearly always an arrangement to observe the preparation first in convergent light and then in parallel polarized light. This change can often be brought about by taking away or adding parts of the condenser. MICROMETRY It is often required in microscopical work to determine the the image correctly to a few thousandths of millimetres, the object itself is measured accurately to some See also:hundred-thousandths of milli-metres, if it has been magnified a hundred times by the objective. To keep up this degree of exactitude the magnification of the objective must be carefully ascertained, e.g. by using an objective micro-See also:meter. A fine scale with known intervals is put on the stage plate, and by determining the distance between the graduations of the objective micrometer formed through the same objective, by means of the screw micrometer ocular, the magnification of the objective is determined. As the errors in the graduation of the objective micrometer are also magnified, very exact scales are necessary. When determining the magnification the microscope must be used under exactly the same conditions: neither the length of the tube nor the focal length of the objective may be altered.

A fixed eyepiece micrometer is simpler and more popular. This consists of a scale on a little glass plate, which, instead of a cross See also:

wire, is placed in the eyepiece. The adjustment must be such that the image produced by the objective falls exactly in the plane of the scale. The size of the image is determined by calculating the entire See also:interval taken up by it. By using an objective micrometer in See also:place of the object, the magnification of the objective can be ascertained and from this the actual size of the object. As fractions of intervals can only be estimated in this method, a measurement with such an eyepiece scale can of course not be as exact as with a screw micro-meter ocular. However, such a determination of size is often quite accurate enough. A third method employs a See also:drawing prism. The object and the Irawing plane are seen at the same time and the outlines can be readily See also:drawn. If, as before, an objective micrometer is placed below the microscope in the place of the object, and the size of a special micrometer-interval is drawn on the same See also:board, then the actual size of the object an be ascertained. Instead of first drawing the object and the objective micrometer, they can of course be projected at the same moment on a scale on the drawing board. The errors attending the determination of the size of a microscopic object depend chiefly on the accuracy of the objective micrometer; any errors, in the micrometer being magnified by the objective.

These may be diminished by using different parts of the objective micrometer for the correction of the eyepiece scale, and the calculation of the size is based on the found mean value. A second See also:

error can arise through the inaccuracy of the eyepiece micrometer, and also in the case of a screw micrometer through periodic faults of the screw, and through dead motion. The eyepiece micrometer allows its errors to be diminished, if one measures at different points and then fixes a mean value. The dead motion of a micrometer screw is best avoided by working the screw always from one and the same side. The thickness of the cross wire may also occasion a See also:fault. For this See also:reason there is sometimes employed two very narrow threads lying beside one another, and which limit the image as nearly as possible.

End of Article: MECHANICAL

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