Page:EB1911 - Volume 18.djvu/429

 up to 0·002 mm. An essential in all rough and fine adjustments is that the motion must always be parallel to the optical axis of the microscope, so that the same point in the object remains in the centre of the field.

Another condition which must fulfilled by a good stand is the power of inclination. It is only rarely necessary to arrange the preparation really horizontal; and for easy observation, especially when it will take a long time, ;it is of great assistance if the microscope 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 microscope can be placed in all positions between the vertical and the horizontal. The horizontal position is sometimes necessary if photographs are to be taken by the microscope.

. 53.—Double Lever Adjustment of Watson & Sons.

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 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. 60, 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 simple microscopes the stage plate lies on the stand held by two springs, and must be moved by the hand (fig. 60, Plate). For elaborate work a so-called cross-table is indispensable. By means of screws the stage plate is movable in two directions at right angles to one another, in the 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 special objects the stand must be expressly made; thus stands with tube carriers very much projecting are made for examining sections of the brain. The petrographical microscope is shown in fig. 61, Plate.

In order to determine the refractive 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 graduation B is used to determine the angle in crystals. To obviate mistakes the optical axis of the microscope 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 focal plane of the eyepiece, at the spot where the real image which the objective forms of the object arises, a glass plate is introduced on which are two fine cross lines or even two very thin threads. The eye-lens can be adjusted for the 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 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 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 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 goniometer eyepiece, in which a 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 to the crystal edge.

For examining preparations in polarized light a polarizer D is introduced in the illuminating apparatus below the 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 case G, which can be put into the tube. The placing of the analyser near the objective has the advantage that the field of View is not restricted, as is the case if the analyser is used above the eyepiece. Nicols’s, Glan-Thomson prisms or similar polarization apparatus are used as polarizers and analysers. Below the analyser G a plate H of selenite or 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 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.

It is often required in microscopical work to determine the size 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 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 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).

. 54.—Screw Micrometer Ocular. Sectional elevation and plan (Zeiss). Directly below the collective lens of a 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 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 division. The calculation is most convenient if the micrometer is 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 measures