706. Focusing

Focusing. When a convenient and satisfactory illuminant has been selected, the student may begin focusing the slide. The principle of focusing is to adjust its relations to the object on the slide, so that a clear image is obtained. Care must be taken in obtaining the focus and a certain line of procedure must be followed, which, by practice, becomes habitual. Exercise care that you do not bring the front of the lens (objective) and cover-glass in violent contact, and always begin focusing with a low (two-thirds) objective, as the working distance between the lens and the slide is greater than with the high-power objectives, and the danger of the objective and cover-glass coming in contact is less likely to occur. With the high-power objectives, in which the working distance is so small that the front of the objective is very close to the cover-glass, there is danger of injuring the instrument as well as the slide.

707. In consideration of the foregoing, and until the student has become thoroughly familiar with these prin-ciples, focusing should be practiced without the camera attachment and with a two-thirds objective, and always keep in mind the rule previously given, never to use more illumination than is necessary.

708. In obtaining the focus, after the slide has been placed upon the stage under the clips to secure it in position, and the objective is in position, if the instrument is supplied with a coarse and fine focusing attachment, lower the tube by the coarse adjustment until the front of the objective is within one-fourth of an inch of the object. Look through the eye-piece and slowly elevate by the adjustment until the image is distinct. The upward movement should be slow, so that if the object is faint it is not missed and you do not run the lens beyond the sharp focus. The object will first appear with faint outlines, then gradually become more distinct, and finally sharply defined; but if the adjustment goes beyond this point, the object will gradually become dim, in which case return to the point of greatest distinctness.

709. In focusing with higher-power objectives, lower the objective by coarse adjustment until the front of the objective is nearly in contact with the cover-glass. Looking into the eye-piece, slowly elevate the tube with the coarse adjustment until the image appears; then, for final focusing, use the fine adjustment.

710. When racking the adjustment to obtain a focus of the image, it is advisable to slightly move the slide or object in different directions while viewing the image through the eye-piece, as the flitting of shadows or colors across the field will indicate when the objective is nearing the focal point. By this means you also select the portion of the field you desire to photograph. With this accomplished, remove the eye-piece and place it within the cone of the camera, first removing the ground-glass and then slipping the eye-piece into the cone from the upper slide. With the eye-piece in position in the cone of the camera, the barrel of the eye-piece will protrude beyond the end. Next place the camera cone over the tube of the microscope, slipping the eye-piece which projects through the camera cone into the barrel of the microscope, attaching the camera over the tube and fitting it flush with the milled ring, which places the eye-piece in proper position. Then very slightly turn the thumb-screw on the camera cone, to secure the camera firmly to the microscope.

711. Where the more expensive regular extension bellows camera is employed, as shown in Illustration No. 127, the eye-piece is not removed from the microscope in order to attach the camera, but, on the contrary, the camera is fitted with a brass hood which fits snugly over the eye-piece of the microscope and telescopes it sufficiently to exclude all rays of light from entering the camera at this connection. Focusing is then proceeded with upon the ground-glass.

712. When focusing the camera, in place of attempting to look through the glass, as you would when focusing the microscope, you observe the image by looking upon the ground-glass, and in order to exclude all light from the surface of the ground-glass, the head and camera should be covered with a focusing cloth, as illustrated in Fig. 1, Illustration No. 126. The image will appear upon the ground-glass screen considerably magnified.

713. In placing the ground-glass into the camera, exercise care that the ground-glass side is downward. This is essential in order to secure perfect focusing and recording of the image upon the plate. If the ground-glass employed is not sufficiently transparent, it may be made more transparent by rubbing the surface with a mixture of equal parts of alcohol and glycerin. This will make the focusing screen very transparent and serve every purpose. In applying the mixture it is advisable to apply it in a circular motion, and only over the center of the ground-glass, covering a space about the size of the image on the glass.