180 Oil Angle. 

 (Numerical Aperture T52.) 



NUMERICAL APERTURE. 73 



again to 180 oil angle, the ratio of opening to power 

 progresses also, and attains its maximum, not in the 

 case of the air angle of 180 (when it is exactly equiva- 

 lent to the oil angle of only 82), but is greatest at 

 the oil angle of 180. If we assume the objectives to 

 have the same 

 power through- 

 out we get rid of 

 one of the factors 

 of the ratio, and 

 we have only to 

 compare the dia- 

 meters of the 

 emergent beams, 

 and can represent 

 their relations by 

 diagrams. 



Fig. 40 illus- 

 trates five cases 

 of different aper- 

 tures of J - in. 

 objectives, viz. : 

 those of dry ob- 

 jectives of 60, 

 97, and 180 air 

 angle, a water- 

 immersion of 180 

 water angle, and 

 an oil-immersion 

 of 180 oil angle. 

 The inner dotted 

 circles in the two 

 latter cases are of 



the same size as 



,i , j of various dry and mmerson 



that correspond- game power ( |. in-) from an air angi* of 60 to an 



ing to the 180 oil angle of 180. 



air angle. 



A dry objective of the maximum air angle of 180 

 is only able to utilize a diameter of back lens equal to 

 twice the focal length, while an immersion lens of even 

 only 100 utilizes a larger diameter, i.e., it is able to 



O 



180 Water Anglo. 

 (Numerical Aperture T33.) 



180 Air Angle. 

 96 Water Angle. 

 82 Oil Angle. 

 (Numerical Aperture I'OO.) 



97 Air Angle. 

 (Numerical Aperture 75.) 



60 Air Angle. 

 (Numerical Aperture '50.) 



FIG. 40. 



Relative diameters of the (utilized) back lenses 

 of various dry and immersion objectives of the 



