ZOOLOGY AND BOTA.NY, MICROSCOPY, ETC. 



817 



heads hi and n. The former movement is made to do duty as a 

 screw-micrometer, having a drum p attached, engraved with 125 

 divisions, each having the value of ■ 002 mm., the screw pitch being 

 • 25 mm. The revolutions of the screw can be read off on an index i. 

 The margin of the stage is graduated and also dentated, so that it can 

 be turned by the finger. For convergent light, two Bertrand lenses 

 of 12 and 8 mm. focal length are placed above the polarizer, and a 

 third above the objective.* 



Fig. 152. 



(3) BecFs Lithological Microscope (Figs. 152-4). — This instru- 

 ment (Fig. 152) is modelled on the plan of the " Economic " Micro- 



* Cf. this Journal, i. (1S7S) p. 292. 



3 l 2 



