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BELL SySTEM TECHNICAL JOURNAL 



is meant the ability to see separate and distinct lines as individual 

 units when these lines are spaced very close together. Resolving 

 jiower or the number of lines ])er inch resoK'ed is expressed numerically 

 by the equation 



in which A' is the nuini)cr nl lines per inch, N.A. is the numerical 

 aperture (defined below) and X is the wave-length in inches. An 

 objective of high resolving power when correctly used will resolve 



Front len& i^ai 

 immfrsion obirrtivf — . 



Front /*»«,T of a dry lens 



n'l.fzo 

 Mounting- Stediu 



Surface of the conden^r- 



Fig. 6 — Diagram illustrating numerical aperture and the superior light gathering 

 powers of an oil immersion objective. 



lines spaced 100,000 to the inch, whereas an objective of inferior 

 resolving power under the same condition will not be able to dis- 

 tinguish the lines as distinct units. 



As will be seen from Fig. 6, an immersion lens has greater light 

 gathering power than a dry lens of corresponding focal length. This 

 light gathering power is expressed as numerical aperture which term 

 in reality supplies a measure of all of the essential qualities of the 

 objective. The magnitude of the innnerical aperture is expressed by 

 the eqtiation 



N.A. = n sin V, 



n being the refractive iiTclex of the medium conlainetl between the 

 cover-glass and the front lens of the objective, and U tiie semi-apertua. 

 angle of the system. 



For a gi\en magnification ami under cimiparable conditions the 

 resolving power is directh' proportional ti_ the numerical aperture. 

 The brightness of the image is proportional to the square of the 



