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. 



xin, 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 \vork there must always 

 be present some form of regulating device like a rheostat or induc- 

 tor (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 lam]) 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, 14-6). 



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 pro- 

 duce so large screen images as with the direct current (fig. 302). 



