NUMERICAL APERTURE 67 



and r as a rule, great working distance and penetration are 

 not required. For the finest resolution of diatoms, T V' 

 oil immersion apochromats with a N.A. of 1-4 can be ob- 

 tained, and Messrs. Powell and Lealand make a T V" of 

 N.A. 1-5, but objectives of such high aperture are so easily 

 damaged, owing to the difficulty of holding the hyper- 

 hemispherical front lens firmly in the mount without 

 cutting down its aperture, that an objective of N.A. 1-2 

 or 1-3 is to be preferred for general work. The N.A. 

 usually associated with objectives of various powers is 

 given in the table on page 81. 



Oil-Immersion Lenses. The formula N.A. = n sin a 

 indicates that with dry lenses the greatest possible 

 numerical aperture is 1, as n for air is 1, and the greatest 

 value of sin a is 1. The maximum reached in practice is 

 about 0-95, for instance, in the Zeiss 4 mm. apochromat. 

 At the same time the formula points to a possible method 

 of increasing N.A. by increasing the value of n, and this 

 is done by making the objective work in a medium of 

 higher refractive index than air. An objective so used 

 is called an oil- or water-immersion lens, according to the 

 medium employed. Cedar oil is the most usual immer- 

 sion fluid, as its index of refraction is practically equal 

 to that of the cover-glass, namely, 1-515, on which account 

 an oil-immersion objective is spoken of as a homogeneous 

 immersion lens. Water and glycerine are also used, but 

 as their indices of refraction are less than that of cedar 

 oil they are not so efficient, and Messrs. Zeiss have intro- 

 duced the use of Napthalene Monobromide (index of ref . 

 1-65) for use with some lenses of extreme aperture, but 

 it is necessary to use special slides and cover-slips made 

 of glass having the same refractive index, otherwise 

 the great N.A. cannot be utilised. These are not much in 

 use. The following diagram explains the superiority of 

 oil-immersion over dry lenses, by indicating how they 

 gather more light from an object than similar dry ones. 



