The Mechanical Parts of the Microscope

THE MECHANICAL PARTS OF THE MICROSCOPE scopic effects can be obtained with these systems by adjusting the eyepieces so that they produce Ramsden circles separated by a distance slightly less than the distance between the eyes of the observer. When the observer's eyes are placed at the Ramsden circles, the light entering the right eye comes mainly from the left side of the object-giass and vice versa. With a little expe rience the adjustment required to give stereoscopic relief can be made without difficulty. This may be done more easily perhaps if capped eyepieces are used in which stops with semicircular apertures are fitted, as in Abbe's stereoscopic eyepiece system. The stops are placed in or just below the Ramsden circles, and are used with the curved sides of the apertures outwards. Without these stops it is easy to produce pseudoscopic appearances by making the distance between the Ramsden circles larger than the inter-ocular distance, so that the light en tering each eye comes mainly from the wrong half of the object-glass. In the Greenhough binocular two separate micro scopes are used, with separate object glasses (fig. 3o). These microscopes are fitted with prismatic erecting systems just below the eyepieces, to produce stereo scopic and not pseudoscopic effects.

The Stage.

The stage for carrying the object is usually fixed rigidly to the micro scope limb, though in some microscopes it is mounted on slides on the limb and is movable by means of a rack and pinion mechanism. The stage may be a simple plane table on which the object is moved about by hand when different portions are to be examined, or it may be made with slides permitting of movement in two di rections at right-angles, and with a bearing to allow of rotation about the axis of the microscope. Centering screws are some times fitted so that the axis of rotation may be brought into coincidence with the axis of the object-glass. The stage should be rigidly supported, and should bear the weight of comparatively heavy specimens without serious deflection. The mechanical slides, and the rotation bearings if fitted, should op erate smoothly but without tendency to slip under the weight of the movable parts, or under the weight of any specimen likely to be examined. The mechanical slides of the stage should be accu

rately at right-angles to the axis of the microscope, as should also be the plane in which the stage moves when it is rotated. Except in some metallographic microscopes, the centre of the stage is cut away, to admit light through to the specimen when illumina tion with transmitted light is required.

The Sub-Stage.

Provision is made below the stage for mount ing the illuminating apparatus required for examining an object with transmitted light. The minimum equipment necessary is a mirror which can be moved into various positions, so as to reflect light up through the hole cut in the stage. In the more completely equipped microscopes special sub-stage attachments are fitted. These are adapted to carry a condenser, polarizing apparatus, various stops, a disk of ground glass and colour filters, with an iris diaphragm to permit of varying the cone of illumination pro vided by the condenser. Geared cells, to permit of rotating various accessories used in the examination of objects under polarized light, such as selenite plates, are often included in the more elaborate sub-stage equipment. The cells used for carrying the stops should be fitted as close as possible to the underside of the condenser, as should also the iris diaphragm.

The sub-stage is usually, but not invariably, carried on a block which slides on the limb of the microscope, and which can be moved up or down by a rack and pinion; the mirror is then mounted on an extension of the limb below the sub-stage slides. The sub-stage condenser fitting should have centring adjustments to allow of the axis of the condenser being brought into line with the axis of the object-glass. The various attachments are generally fitted so that they can be conveniently swung out of position when not required. In most instruments the whole sub-stage can either be swung bodily out of the axis or removed entirely. The sub stage is often mounted on a pillar attached to the underside of the stage, and is movable up or down on a spiral cut on this pillar. In some instruments there is no proper sub-stage at all, any con denser or other illuminating apparatus being made to slip into a tube fixed under the stage.