CH, I] MICROSCOPE AND ACCESSORIES 



23 



dry objectives this is air and n=/, for water immersions n=/-3j, and for 

 homogeneous immersions n=/.jz. (For a table of natural sines, see third 

 page of cover.) 



Objective 



•It; 8 



Natural Sine 



of half the angular 



aperture 



(sin u. ) 



Index of 

 Refraction 

 of the medi- 

 um iu front 

 of the objec- 

 tive («) 



Numerical Aperture 

 (N. A. )=n sin u 



25 mm, 

 Dry. 



25 mm. 

 Dry. 



12}4 mm. 

 Dry. 



12^ mm. 

 Dry. 



6 mm. 

 Dry. 



6 mm. 

 Dry. 



3 mm. 

 Dry. 



3 mm. 

 Dry. 



2 mm. 



Water 



Immersion 



2 mm. 



Homogeneous 



Immersion 



2 mm. 



Homogeneous 



Immersion 



4o" 



42° 



75° 

 136° 



1 15° 



163 



96°I2' 



20 



Sin =0.1736 



2 



40 



Sin =0.3420 



2 



42- 



Sin =0.3584 



2 



100 



■ Sin =0.7660 



2 - 



7S 



Sin =0.6087 



2 



136 

 Sin =0.9272 



2 



115 



Sin =0.8434 



2 



163 



Sin =0.9890 



2 



9 6°I2' 



Sin =0.7443 



iio 38' 



iio°38 / Sin -=0.8223 



2 



i34°io' 



Sin- 



=0.9211 



»=i-33 



«=1.52 



=1.52 



N.A.= iXo- 1 736= 0.173 

 N.A.= 1X0-3420=0.342 

 N.A.= 1X0.3583=0.358 

 N.A.= 1X0.7660=0.766 

 N.A.= 1X0.6087=0.609 

 N.A.= 1X0.9272=0.927 

 N.A.= 1X0.8434=0.843 

 N.A.= 1X0.9890=0989 

 N.A.=i.33Xo. 7443=0.99 

 N.A.=i. 52X0. 8223=1. 25 

 N.A.= 1. 52X0. 9210=1. 40 



§ 40. Significance of Aperture.— As to the real significance of aperture in 

 microscopic objectives, it is now an accepted doctrine that — the corrections in 

 spherical and chromatic aberration being the same— (1) Objectives vary 



