CH. IX] 



PROJECT/ON MICROSCOPE 



255 



condense it either upon the object for lantern slides and low objectives, 

 or it narrows the light into a cone of the proper size for further con- 

 centration by the achromatic condenser (§ 412). One of the most 

 used, and also one of the best condensers for the arc lamp is composed 

 of two plano-convex lenses with their convex sides facing each other. 

 There is then one plane face next the radiant and one toward the mi- 

 croscope. The lens next the radiant is somewhat smaller than the 

 other. Both are loosely mounted to allow for expansion and the cell 

 in which the} 7 are mounted should be freely ventilated. This con- 

 denser should be adjustable back and forth and up and down. 



Fig. 206. 



Fig. 206. Arrangement and Centering of the Radiant {Leiss). 



In {/) The radiant, i. e., the crater {Fig. 205) is too far to the right ; 



(2) The a'ater is too far to the left ; 



( 3 ) The crater is too high ; 

 {4) The crater is too low ; 



( 5) The crater is too far from the lamp condenser ; 



(6-7) The crater is too near the condenser. 



(8 ) The crater is in the correct position . 



As pointed out in the explanation of Fig. 205, there may be a slight central 

 shadow with soft cored carbons when the lamp and condenser are in the best relative 

 position. 



§411. "Water Bath for Removing Heat. — This is a cell of 

 some kind with plane glass faces. It should be approximately of a 

 size to allow the light to pass through a stratum of water 50 mm. 

 thick. Even this will allow something over 2% of the heat to pass. 

 It is a great advantage to have cool or cold water circulate through this 

 constantly. 



