ZOOLOGY AND BOTANY, MICKOSCOPT, ETC. 1023 



ness of the cover-glass as well as the tube-length has an important in- 

 fluence 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 

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

 unadju-table objectives are adjusted varies with the different opticians, as 

 shown in the table below. The information in the table was obtained by 

 direct inquiry as for the information concerning ' tube-length ' hereinafter 

 mentioned.' * 



Table showing the Thickness of Cover-glass fcyr which unadjustable objectives 

 are corrected hy various Opticians. 



fJ. Green, Brooklyn. 

 J. Grunow, New York. 

 Powell & Lealand, London. 



H. E. Spencer & Co., Geneva, New York. 



W. Wales, Xew York. 

 0*18 mm. Klonne & AI tiller, Berlin. 

 0-17 „ E. Leitz. Wetzlar (when tube 160-170 mm.) 



0-16-25 „ Boss & Co., London. 

 0-16 „ Bausch eV Lomb Optical Co., Eochester. 



•15—20 „ (16 mm. apochromatic oil-immersions), C. Zeiss, Jena. 

 0' 15-18 „ C. Eeichert, Vienna. 



Gundlach Optical Co., Eochester. 

 0-15 ., <; W. & H. Seibert, Wetzlar. 



E. it J. Beck, London. 



J. Zentmayer, Philadelphia. 



Xachet et Fils, Paris. 



Bezu, Hausser et Cie, Paris. 



Swift & Son, London. 



A uniform thickness of cover-glass for unadjustable objectives seems 

 also desirable ; then by the use of some cover-glass measure, like the one 

 made by Zeiss, the microscopist could select covers of the proper thickness 

 to be used for the specimens to be studied with unadjustable objectives." 



Objectives. 



[" An optical firm offers for sale ' homogenerotis ' immersion objectives."] 



Queen's Micr. Bull., IV. (1SS7) p. 39. 

 Pelletax, J. — Les Objeetifs. (Objectives.) 



Journ. de Microgr.. XI. (1SS7) pp. 446-8. 476-81 (in part). 

 Boss, "^V. A. — New Optical Substance for Objectives of Microscopes, &c. 



[" A transparent substance (it is not glass, for no alkali is employed in its manu- 

 facture)" having "a hardness and specific gravity equal to that of emerald, 

 whilst its refractive index is obviously very high." And reply by F. H. Wenham 

 that he has •' seceded from the ranks of the ' Diatomaniacs,' and ceased to take 

 anv interest in dots and strife, and it is not probable that I shall ever again 

 work at the Microscope or its appliances."] 



£n,]l. Mech., XLYI. (1887) pp. 278 and 301. 

 ScHTLZE. A. — On Abbe's Apochromatic Micro-objectives and Compensating Eye-pieces, 

 made of the new optical glasses in the works of Dr. Carl Zeiss in Jena, with some 

 general remarks on object-glasses. 



Proc. Phil. Soc. Glasgow, XVIH. (1887) pp. 28-40. 



Cf. infra, p. 1029. 



