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280 HIGH POWER MICRO-PROJECTION [Cn. IX
entire beam of light from the lamp condenser may enter. Then, just as in ordinary observation, one can often make the contrast more striking by cutting down the aperture somewhat by closing more or less the substage condenser diaphragm. It must not be cut down too much, for that will render the image dim and defeat the very purpose of the substage condenser.
As a general statement, much more of the aperture of the objective can be used in projection than in ordinary direct observation in the microscope. Naturally, objectives of relatively large aperture give the more brilliant images (see § 855).
§ 405. Oculars to use in projection. — Generally speaking, only low powers are used (x2, X4, x8). The lower the power the more brilliant the image. Compensation oculars have been found better than the Huygenian. A compensation ocular as high as xi2 gives brilliant images for short screen distances.
One should not forget that the ocular, when used in projection, is really a second projection system, and hence the image will be erect on the screen (fig. 207).
§ 404a. Centering the substage condenser. — As the substage condenser becomes one of the optical elements in projection, its principal optic axis must be centered on the common axis of the entire apparatus.
It is assumed that the microscope without the substage condenser has been properly centered as directed in § 374-375.
To center the substage condenser, use the ocular and objective (x4 ocular, 8, 10 or 1 6 mm. objective), remove the bellows if present (fig. 133), place a piece of white cardboard at about 45 degrees as shown in fig. 1 16, between the large condenser and the substage condenser, and light the cardboard well with a mazda lamp. This will give the light for the microscope.
Now put a preparation on the stage and focus the microscope as for ordinary observation. Remove the specimen and close the substage iris diaphragm nearly up. With a pocket magnifier examine the eye-point or Ramsden's disc (fig. 127 E P) beyond the ocular. This disc of light appears as if on the back lens of the objective. If the iris is properly made and the substage condenser is centered with the objective and ocular, the center of light will appear to be exactly in the middle of the back lens of the objective (fig. 151). If the substage is not in the optic axis then the disc of light will appear eccentric; and if the substage condenser is markedly off the center the spot of light will make a break in the black ring on one side as shown in fig. 30, 1-4. If it is only slightly off center, the disc of light will seem to be surrounded by a dark ring of unequal width. If the substage condenser is not found to be correctly centered, the centering screws (fig. 150) must be used to move it slightly until the disc of light is central as shown in fig. 151.
The Abbe condenser found on most microscopes has no centering screws. The makers center the instrument carefully and fix it in position. If it is found badly out of center it is best to return it to the makers for adjustment.