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284 USE OF ALTERNATING CURRENT [Cn. IX
water in which there are many large bacteria and infusoria, a most striking picture on the screen is made. For this projection a water immersion is excellent. An oil immersion may also be used and also a dry objective of 4 to 6 mm.
For securing a large field, the objective and amplifier are better than an objective and ocular (§355).
USE OF ALTERNATING ELECTRIC CURRENT WITH THE PROJECTION
MICROSCOPE
§ 412. It is unfortunate that it should ever be necessary to use alternating current in micro-projection ; but if that is all which can be obtained, much can be accomplished with it by skillful handling.
(For a discussion of the difference between direct and alternating current and the relative amount of light yielded by the two, also for the possibility of getting direct from alternating current by means of a motor-generator set, or by a ' 'current rectifier," see Ch.
xui, § 681-683, 751-752).
§ 413. Wiring the Arc Lamp. — This is exactly as for the magic lantern, (fig. 3). And as with all arc lamp work there must always be present some form of regulating device like a rheostat or inductor (fig. 145, 197, § 748).
§ 414. Arrangement of the carbons. — For micro-projection the carbons should always be at right angles, and the light will then be almost wholly from the upper or horizontal carbon (fig. 191). As this is in the optic axis and looks directly toward the condenser it is the most satisfactory source of light available with this as with the direct current lamp for micro-projection. This is because the image of the crater of one carbon is as large as can be received by the projection objective.
It is especially necessary for micro-projection that the lamp have fine adjustments to keep the crater exactly centered (fig. 3, 146).
§ 415. Amount of current necessary. — As the alternating current gives less than one-third as much available light as the direct current one cannot project with such high powers nor produce so large screen images as with the direct current (fig. 302).