252 



PROJECTION MICROSCOPE 



[CH. IX 



In order to see whether the carbons are in the most favorable posi- 

 tion, Barnard and Carver (J. R. M. S.. 1898, p. 171) used a pin-hole 

 camera at right angles to the carbons. This camera has a ground 

 glass with cross lines to serve as guides in maintaining the proper 

 position of the carbons. 



Fig. 204. 



Fig. 205. 



Figs. 204-205. Front and side views of the carbons of an arc light to give the 

 best illumination. -\- and — indicate the positive and negative poles 



Fig. 204 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 pos- 

 itive ( + ) or upper carbon. The carbons have soft cores. 



Fig. 205 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 the 

 special achromatic condenser next the object (§ 399, 4). 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 lower 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. 



% 405. Positive and Negative Carbon. — The mechanism of 

 the lantern is arranged so that the upper carbon should be positive 

 (+). In setting up the apparatus one may not be able to insert the 

 wires correctly at first. All one has to do is to make the connections, 



