392 The Microscope. 



So long as no common standard is employed, it seems to the 

 writer that every objective should be accompanied by a statement 

 and a diagram indicating the tube-length in millimeters for which it 

 was corrected, and showing also the parts of the microscope included 

 in this measurement. If the objective is unadjustable, a statement 

 should also accompany it, giving the thickness of cover-glass for 

 which it was adjusted. (See below under II). 



II. The thickness of cover-glass for which unadjustable 

 objectives are corrected. 



As the thickness of the cover-glass as well as the " tube-length " 

 has an important influence on the perfection of the microscopic 

 image, and as almost all objects for microscopic examination are 

 covered, the objective must be adjustable to compensate for the 

 various thicknesses of cover-glasses used, or some uniform thickness 

 of cover-glass must be selected, for which the optician coiTects or 

 adjusts the objective once for all. The thickness for which such 

 unadjustable objectives are adjusted varies with the different opti- 

 cians, as shown in the table below. The information in the table 

 was obtained by direct inquiry as for the information concerning 

 " tube-length." 



Table showing the thickness of cover-glass for which unadjust-1 

 able objectives are corrected by various opticians: 



(J. Green, Brooklyn. 

 J. Grunow, New York. 

 ,- "p mm. \ Powell and Lealand, London. 

 " } H. E. Spencer & Co., Geneva. N. Y. 



\ W. Wales, New York. 

 -^\ mm. Klonne und Milller, Berlin. 



^^0 mm. E. Leitz, Wetzlar. 



"fro*"^^- Ross & Co., London. 



Y^o^ mm. Bausch & Lomb Optical Co., Rochester. 



1^2^**- mm. (j'o^y^mm. apochromatic oil immersions), C. Zeiss, Jena. 

 if^y mm. C. Reichert, Vienna. 



( Gundlach Optical Co., Rochester, 

 y^o mm. -^ AV. & H. Siebert, Wetzlar. 



( R. & J. Beck, London, 

 ly^y-mm. J. Zentmayer, Philadelphia. 

 i-«^-i|.H I Nachet et Fils, Paris, 

 mm. I Bezu, Hausser et Cie, Paris. 

 yLQj, mm. Swift & Son, London. 



