with our lenses, of immersion fluids derived from other sources, as 

 the use of fluids of irregular refractive and dispersive power may 

 entail considerable impairment of the optical performances of the ob- 

 jectives. 



a b 



Fig. 2. Cap-bottle for Immersion-oil. ('/, Full Size.) 



The two little bottles shown in Fig. 2 a and 2 b afford comfort and convenience in 

 using the immersion oil. The cap and rod of the first-mentioned bottle are made of glass, 

 those of the second, which we originally made on specifications supplied by Dr. L. MACH 

 (see "Zeitschr. f. wiss. Mikrosk.", Vol. XIV, 1897, pp. 348350), consist of metal. 



Code-words : 



Price of bottle with glass cap and rod : Marks i. . Pabellon. 



Dr. L. MACH's pattern ...... 7.50. Pabulo. 



With each homogeneous immersion objective of our own make 

 trc supjtly gratis one of MACH's bottles with a separate flask con- 

 taining about 15 yr. of Cedar-wood Oil. 



The expression "Numerical Aperture", or "Aperture" simply, was intro- 

 duced in 1873 by Professor ABBE on the basis of his own theoretical investi- 

 gations. The value of the numerical aperture is determined by the equation: 



num. Ap. n. sin u, 



n being equal to the refractive index of the medium situated between the cover- 

 glass and the front lens of the objective, u representing half the angle of aperture. 



The numerical aperture of a lens determines all its essential qualities: the 

 brightness of the image increases with a given magnification, other things being 

 equal, in proportion to the square of the aperture; the resolving and de- 

 lining powers are directly proportional to it; the power of differentiating 

 depth (the focal depth) is inversely proportional to it. (For references to the 

 subject see ABBE on "The Estimation of Aperture" in the Journ. of the R. 

 Microsc. Society, Series 2, Vol. I, 1881, pp. 388423, and CZAPSKI'S "Theorie 

 der optischen Instrumente", Breslau, 1893, p. 224.) 



The mounts of all objectives are provided with the English standard screw 

 of about 20 mm ( 3 / 4 in.) external diameter. 



