88 PHOTOMICROGRAPHY 



desired gap between them is maintained with great 

 accuracy. 



The whole of the lamp is enclosed, the front (4) carry- 

 ing a small aplanatic condenser with a N.A. of 0-4, and 

 an iris diaphragm. There is also a dark glass window 

 through which the arc can be examined. 



A lamp of similar appearance is made for hand regula- 

 tion, but the carbons are arranged at right angles to 

 each other. Either lamp can be carried on a saddle stand 

 for the optical bench, but that illustrated is fitted with 

 levelling screws (3) for centring the crater. 



Hand-feed Arc Lamps. A periodic adjustment of the 

 carbons by hand does not give so regular a feed as an 

 automatic control ; but the mechanism on many of the new 

 hand-feed lamps, actuated by a milled head, works very 

 smoothly, and despite the constant attention necessary 

 to keep the arc regular, these lamps are perfect!} 7 satis- 

 factory for most purposes. Their simplicity enables 

 them to be made at a much lower price than a cor- 

 responding automatic lamp, and they are at present 

 undoubtedly one of the most popular illuminants obtain- 

 able for photomicrography. They usually run on about 

 4 amperes current, and so can be used with safety on any 

 ordinary wiring. With suitable adjustments and resist- 

 ances they can be used either on direct or alternating 

 supply, and give from 300 to 500 candle-power. 

 f" The " Liliput " lamp of Leitz is shown at Fig. 1, while 

 Baker has more recently introduced a somewhat similar 

 lamp. Both of them carry a bull's-eye condenser, 

 focussing in a sliding mount and a hood which surrounds 

 the light. In Baker's an image of the carbon is projected 

 through a pin-hole on to a ground-glass disc for examina- 

 tion ; the Liliput is fitted with a dark blue glass window. 

 Cored carbons are used, arranged at right angles to each 

 other. A single milled head actuates the feed in both 

 lamps ; in Baker's model it is so geared that the horizontal 



