ZOOLOGY AND BOTANr, MICROSCOPY, ETC, 



155 



diaphragm, and tube-length. The focusing differences are always to be 

 multiplied with this factor in order to obtain the true depth (thickness) 

 of the layer. 



As an example : — Objective DD Zeiss, diaphragm 8 mm. diameter, 

 tube-length 155 mm., and four cover-glasses, the thicknesses of which, 

 already ascertained, are 0'146, 0*168, 0*187, 0*22. The focusing 

 ditferences marked by the head of the fine-adjustment screw were 35, 

 40, 45, 52 divisions. Then the reduction factors in 1/1000 [x are 



1^-4*17 1^^ = 4.20 1^-4*16 

 35 ' 40 ' 45 ' 



220 



■52 



= 4*23, 



or on the average 4*19, say 4*2. If the thickness of these cover- 

 glasses had not been known, but the focusing differences had been 

 obtained and multiplied by 4*2, the results would have been • 147, 

 0-168, 0*189, 0*218, instead of 0*146, 0*168, 0*187, 0*22. Differ- 

 ences of -f- * 001, 0*0, -}- * 002, - • 002 ; a result more than 

 sufficiently accurate for the purpose. 



(6) Miscellaneous. 



Garbini's small Steam-generator for Microscopical Technique.* — 

 Dr. A. Garbini describes a small steam-producing apparatus which he 



Fig. 35. 



uses in microscopical technique, especially where paraffin [and gelatin 

 are required. 



* Zeitschr. f. Wiss, Mikr., v. (1888) pp. 168-71 (1 fig.)- 



