ARC LAMPS 87 



positive, so that the crater is drawn forward towards the 

 microscope. 



There must be some arrangement to maintain the 

 correct gap between the two carbons ; it may be auto- 

 matic or depend on adjustment by hand. The automatic 

 is undoubtedly the better, provided the mechanism 

 works smoothly ; and a lamp so fitted should give a 

 constant light, and require no further attention when 

 once set in the optical axis of the microscope. 



Automatic Arc Lamps. A description and figure of 



8 



Fig. 24 

 ZEISS' 5-AMi'i:i5E AUTOMATIC AJ;C LAMP 



Zeiss' 5-ampere automatic arc lamp is given as an 

 example of the type of lamp best adapted for photo- 

 micrography. Siemens' Al Carbons are used, inclined 

 to each other at an angle of about 60. The horizontal 

 positive carbon is cored, and has a diameter of 5 mm., 

 the negative is solid and of 4 mm. diameter. After 

 careful adjustment by the milled head (2) the carbons 

 are regulated by clockwork mechanism wound up by 

 the screw (1). This causes a slight forw r ard move- 

 ment of the carbons, every twenty seconds, and the 



