60 Modern Microscopy 



immersion medium is not of the same density as the cover- 

 glass a correction collar is essential, but there are subjects 

 with which oil could not be suitably used, and in such 

 cases the water immersion lenses have to be chosen. 



Mono-bromide of naphthalin is, at present, only used with 

 one form of objective, a xc-inch, by Carl Zeiss, of Jena, 

 having a numerical aperture of 1"63. The refractive index 

 of this medium is 1'657, and special flint cover-glasses of the 

 same density have to be employed with it. This restric- 

 tion, together with its high price — £40 — has prevented its 

 being largely used. Those who have had an opportunity 

 of working with one have spoken in high terms of the 

 beautiful effects it yields. 



It must be borne in mind that objectives that are in- 

 tended to be used immersed are specially corrected for the 

 specific medium to be employed. Ordinary lenses intended 

 for use dry cannot be advantageously worked immersed. 



APERTURES OF OBJECTIVES— ANGULAR 

 AND NUMERICAL. 



For reasons which will be stated hereafter, it will be 

 seen that on the aperture possessed by an objective depends 

 the fineness of detail that it is capable of delineating — that 

 i^, the number of lines per inch that it will separate. 



Angular Apbeture. — Before the introduction of immer- 

 sion objectives the ability of an objective to resolve fine 

 structure was known to be dependent on the angle formed 

 by the extreme rays issuing from the object that could 

 be received by the objective. This, which was called the 

 angular aperture of a lens, is, in other words, the angle of 

 the cone which envelops the pencil of light that is received 

 by the objective from a point on the object. 



As we have stated in the description of immersion lenses, 

 an oil immersion lens receiving light at 82°, and a water 

 immersion, receiving light at 96°, would each divide as 

 many lines to the inch as a dry lens having the limiting 



