{ 8 ) 

 I. Numerical Aperture Table. 



The " ArRRTCHF," of an optlwl inatrutneiit liidiciiteB Its greater or Icga capacity for rccclvlnp; rays from the < 

 tnuisnilttlng Ibcm to tlic iraa^e, ainl the aperture of a Microscope oMji-clive is tlHTrfun- Jeteniiiurd t.y 

 I)>-twi'iii ii« fiK'al li'M)»ili hihI the diiiiniiier ot tin- omorp^nt pencil at t'.ie plane of its i mergencL — tbut is, \ . 

 iliaiueter of a siiigl.-lens ol>Joc!ive or of the back leu3 ot acompouMcl objective. 



This ratio is cxpres*-.) for all media and in all casss by n sin tt, « bciiv; tin- refractive Index of the niodhim 

 »'.:nil-ang!e of apnrtiire. The value of n sin u for any pirticuUr tasc is the "numerical aperftire" of th 



The apertures of four otgectives, two of which are dry, one watcr-lmniersion, and one ofl-immers 

 <- compared on the angular aperture view as follows :— 106° (air), 157° (air), I4'i° (water), 130° (oil). 



Example.' 



would be compared 

 Their actual apertuics are, however, as 



nrrmericAl apertuiep 



l'3« orti 



