1 1 



Each time after use the objectives should be made scrupulously clean. The oil adhering should 

 be soaked up with linen or blotting paper, and then the front lens together with its mount should be 

 most carefully wiped dry with a piece of very soft linen moistened by a drop of benzine or xylol. 



~ 



a b 



fig. 3. Cap-bottle for Immersion oil No. 2005 and 2010.' (V 2 Full Size.) 



The two little bottles shown in Figs. 3 a ,. and 3 b afford comfort and convenience in 

 using the immersion oil. The cap and r o'd of the bottle, No. 2005, are made of glass, 

 those of the No. 2010, which we originally made on specifications supplied by Dr. L. MACH 

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



With each homogeneous immersion objective of our own make 

 we supply gratis one of MACH's bottles with a separate flash con- 

 taining about 15 f/r. of Cedar-wood Oil. 



For prices, ordered independently, see below. 



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 n, 



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 resolYing and de- 

 fining powers are directly profportional to it'; the power of differentiating 

 depth (the focal depth) is, for the same object medium, proportional to the co- 

 tangent of half the angle of aperture. (For references, see ABBE, Gesammelte 



&azt 



