Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine (1914)

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278 HIGH POWER MICRO-PROJECTION [Cn. IX brilliant pictures can be produced by using also a parallelizing lens as indicated in § 402 A. If one has an optic bench apparatus (fig. 121, 158, 159) one can get good results with the condensers of all foci by placing the microscope so that a diverging cone of light enters the substage condenser (fig. 1546). It will then be necessary to lower the substage condenser slightly for the higher powers. § 403. Kohler method of using the substage condenser. — The general principle is shown in fig. 170. The microscope is moved toward the main condenser until the focus is at the iris diaphragm. One can tell when the main condenser is focused on the iris diaphragm in the same way as that in focusing on the black hood of the objective (§ 375) viz., by noting when the image of the crater and the tip of the lower carbon appear on the iris. After the image is focused on the iris diaphragm the iris is opened to admit the cone of light, and the substage condenser is raised or lowered slightly to get the most brilliant light. As one can see by the diagrams of light cones and the plates of the light rays and the light cones, the light is diverging beyond the focus so that diverging and not parallel light enters the substage condenser. As the condenser cannot focus diverging light at the same level that it would focus parallel light it may be necessary to lower the substage condenser somewhat to get the most brilliant image with high powers. Furthermore, if a concave lens of 10 to 20 diopters is put in the fork as described in § 402 A the image will be markedly brighter unless a very long focus main condenser is used (fig. 171). (See also Ch. XIV, § 864). § 404. Aperture of the substage condenser. — The purpose of the substage condenser in projection, as in direct observation with the microscope, is to increase the aperture of the illuminating cone. And as it is now one of the fundamental doctrines, that the resolution or making visible of minute details depends directly upon the aperture of the objective used, naturally as much as possible of the aperture of the objective is employed. For this, the substage condenser diaphragm should be wide open, so that the