99 



For thin covers the lenses are separated, or the 

 adjustment is open. 



It is a misfortune that cover glasses are not 

 made of the same thickness, but the difficulties in 

 producing them of equal thickness are almost un- 

 surmountable, and it is also to be deplored that 

 opticians have not agreed upon a definite thickness 

 to which they correct the objectives ; but since 

 this is not the case, all aid should be given to the 

 microscopist to obtain the best possible results. 

 The list herewith given, which has been prepared 

 by Prof. Gage will show the variations in thickness 

 used by different opticians as standard. 



J. Green, 



J. Grunow, 



Powell & Lealand, 



Spencer Lens Co., 



W. Wales. 



Watson & Sons, 



Klonne & Muller, 



E. Leitz, 

 R wink ' elj 



Ross & Co, 



Bausch & Lomb Optical Co, 



c - Zeiss > 

 C. Reichert, 

 Gundlach Optical Co, 

 W. & H. Seibert, 

 R. & J. Beck, 

 J. Zentmayer, 



Nachet et Fils, 

 Bezu ^ Hausser et Cie? 



Swift & Son. 



mm. 



mm. 

 mm. 



mm. 



TV 6 o-T 2 oV mm. 

 Y 1 ^ mm. 



To 5 o-T 2 (ro mm - 

 mm. 



mm 



mm. 



i o _ 1 5 

 VTJO- TO o 





