352 



SUMMAEY OF CURRENT RESEARCHES RELATING TO 



arrangement is placed at the outer end of this tube. As already 

 mentioned, when fresh carbons are inserted they must be allowed to 

 burn off for one to two minutes before replacing the front and filter. The 

 length of the arc should be about 10 mm. ; the horizontal carbon should 

 be the positive. The current is introduced by two binding-screws 



Fig. 70. 



below the lamp-case (fig. 69) If the lamp is used without a cooling- 

 arrangement, the current should not exceed b amperes. In this form 

 the lamp may be regarded as transportable (fig. 70), and may be moved 

 about for the -examination of such articles as mineralogical collections. 

 For the examination of small objects and surfaces it is best to adopt 

 the arrangement of hg. 68, when the current may vary from 3-10 amperes. 



