It is a misfortune that cover glasses are not of uni- 

 form thickness, but the difficulties of producing them 

 of equal thickness are almost unsurmountable, and it is 

 also to be deplored that opticians have not agreed 

 upon a standard thickness for which they correct their 

 objectives. The list herewith given, which has been 

 prepared by Prof. Gage, will show the variations in 

 thickness used by different opticians as standard. 



r 



mm. 



T 2 oo mm. 



TITO mm. 



iVo mm. 



nro-iV'o mm. 



To-T 2 oo mm. 



ToViTrV mm. 



rV'o mm. 



To 2 o-io mm. 



ToVrVo mm. 



i 1 .-) ,,- mm. 



J. Green, 



J. Grunow, 



Powell & Lealand, 



Spencer Lens Co., 



W. Wales, 



Watson & Sons, 



Klonne & Muller, 



Bausch & Lomb Optical Co., 



E. Leitz, 



R. Winkel, 



Ross & Co., 



C. Zeiss, 



C. Reichert, 



Gundlach Optical Co., 



VV. & H. Seibert, 



R. & J. Beck, 



J. Zentmayer, 



Nachet et Fils, 



Bezu, Hausser et Cie., 



Swift & Son. 



9 1 



