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-projec- 

 tion 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. 24, 25 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 substagc condenser. 



364. Water-Cell to prevent overheating. For micro-projec- 

 tion a water-cell in connection with the large condenser is a neces- 

 sity. It absorbs most of the radiant energy in the infra-red part 



