PROJECTION MICROSCOPE 



{_CH. IX a 



arrange the carbons until the best crater is obtained. (See Figs. 

 205 K, 207). One must always remember in working with the lan- 

 tern or projection microscope that the crater should be in the upper 

 carbon (See Fig. 207 and 406). 



405. Kind of Carbons to Employ. The so-called soft cored 

 carbons are the most satisfactory. The crater does not shift around 

 on the end of the carbon so markedly as with hard carbons. The soft 



FIG. 207 



FIG. 207. Front and side views of the carbons of an arc light with inclined 

 carbons, -j- and indicate the positive and negative poles. Compare Figs. 204, 

 206. 



A is a side view showing the carbons in section at an angle of 30 degrees from 

 the vertical and the negative ( ) or lower carbon slightly in front of the positive 

 ( _|_) O r upper carbon. The carbons have soft cores. 



B is a front view of the carbons as seen projected on the screen with a 42 mm. 

 objective. It is a projection of the real image of the carbons formed by a special 

 achromatic condenser next the object. This figure shows that the source of light 

 is the crater in the positive (-f) or upper carbon; it shows also that the lower 

 carbon is slightly below the upper carbon as well as slightly in front. This avoids 

 a shadow from the lower carbon. 



In the center of the crater is shown a slight shadow. This is due to the pit 

 formed in the soft core of the carbon. 



