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CH. IX] MICRO-PROJECTION WITH DIRECT CURRENT 237
cated on the dial of the rheostat. When current is drawn from an overloaded power line, however, the voltage fluctuations are often so great that an ammeter, as well as an adjustable rheostat, is a necessity.
§ 362. Fine adjustment for the arc lamp. — For micro-projection it is absolutely necessary to have fine adjustments on the arc lamp so that the position of the crater can be changed slightly during an exhibition. In the burning of the carbons there is a slight shift in position of the crater even with soft-cored carbons. The crater may be in perfect alignment to start with, and by the shifting as the carbon burns away it may get far enough outside the longitudinal axis on which the apparatus is placed to spoil the light on the screen. This is emphatically true for high powers (16 mm. and higher). If now there are fine adjustments on the lamp (fig. 3, 146), by which the crater can be slightly raised or lowered or turned toward the right or left, compensation for this shifting can be made, and the most brilliant part of the crater kept strictly in the axis where it must be to give satisfactory illumination. Furthermore, it is necessary to have an independent adjustment for one or both of the carbons, so that one or both carbons may be moved independently. This is because the carbons are liable to wear away somewhat unequally, and some one of the mal-positions shown in fig. 2 4, .2 5 would occur if the carbons were not adjustable.
§ 363. Condenser. — The triple form with a meniscus next the radiant (fig. 121, 132) is very satisfactory for micro-projection, although many use the double form (fig. 146) with success. As the objectives used for projection with the microscope are of short focus and rather large aperture the final element of the condenser used to bring the light to a focus should not be of too great focal length. A focus of 150—200 mm. (6-8 in.) is a good average for the condenser with the objectives usually employed (125 to 4 mm., § 355) See § 401 for condenser with substage condenser.
§ 364. Water-Cell to prevent overheating. — For micro-projection a water-cell in connection with the large condenser is a necessity. It absorbs most of the radiant energy in the infra-red part