CH. IX] USE OF PROJECTION MICROSCOPE 251 



381. Screen image of the carbons. One of the good ways of 

 learning to get the carbons in the correct relative position is to 

 study their image on the screen. For this use an objective of 

 50 or 100 mm. focus. By moving the objective somewhat 

 beyond the focus of the condenser an image of the burning car- 

 bons will be projected on the screen and one can tell the exact 

 appearance of the crater and the relative position of the car- 

 bons. The glowing upper carbon ought to show the crater 

 well and appear to face directly toward the observer. As this is 

 an image of the real image of the carbons formed by the condenser 

 the screen image will appear right side up. If the negative or lower 

 carbon is not in the correct position it will shade the image (see 

 fig- 24, 25). 



382. Centering the light and getting the objective at the 

 correct distance from the condenser for an exhibition. In using 

 any of the objectives on the revolving nose-piece it is always to be 

 kept in mind that the centering is most easily accomplished by 

 drawing the objective toward the condenser until the image of the 

 crater and the tip of the negative carbon appear in the opening and 

 upon the objective hood (fig. 140). 



Now if this image is not so that the brightest part is over the 

 opening in the objective hood, use the fine adjustment of the arc 

 lamp and get the image of the crater directly in the opening. The 

 screen image will then be evenly and brilliantly lighted. In case 

 one side is more brilliantly illuminated than the other, one can 

 make the illumination even by the fine adjustments of the arc lamp 

 (fig. 3, 146). 



One can sometimes improve the illumination slightly by looking 

 at the screen image and moving the microscope slightly nearer or 

 farther from the condenser, but as a rule, when the image of the 



tivc carbon was 13 mm. in diameter and solid. In Ewon's lamp the upper or 

 positive carbon is eored and 18 mm. in diameter; the lower carbon is 12 mm. in 

 diameter and solid. 



Experience leads us to recommend cored carbons below as well as above. 

 For the size of carbons for different amperages see 377, 753a. 



For alternating current both carbons are of the same size, and most workers 

 recommend that thev be cored. 



