ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



273 



socket and body-tube, forming a fine-adjustment. 

 A coarse-adjustment is formed by the collar not 

 being attached directly to the pillar, but to a 

 tube sliding in it, wbich can be raised and lowered 

 and clamped by a clamp screw. Tbe eye-piece 

 micrometer is somewhat peculiar, every alternate 

 line of the principal set of 24 lines being num- 

 bered from to 11, with longer lines at 1, 6, and 

 11. In the middle of their length the 

 between the principal lines are redivided into 

 5 subdivisions. 



Ahrens' Giant Microscope. — The object of 

 Mr. C. D. Ahrens in constructing this instrument 

 was to have a Microscope with an exceptionally 

 large field for use with his new form of polarizer.* 



The body-tube is 4|^ in. in diameter at the 

 top, and below the field-lens it cones down to 

 the nose-piece ; it has two attachments, one by 

 screws to the top of the stem and tail-piece, and 

 another at the nose-piece, where it is attached 

 to a short bar screwed to the stem. The stage 

 racks on the stem which ends in 

 a cross-piece which carries a short 

 tail-piece on which the mirror 

 socket racks. The pillar rotates 

 on the tripod. 



The achromatic eye-lens is 

 If in. in diameter, and the field- 

 lens 3f in., the foci being respec- 

 tively 3 in. and 6 in. A diaphragm 

 is placed in the focus of the eye- 

 lens. 



Mr. Ahrens considers that the 

 defects in flatness of field f and 

 marginal colour referred to when 

 the Microscope was exhibited 

 were due to the objective used. 



* See infra, p. 276. 



t See this Journal, 1888, p. 1066. 



