774 



HANDBOOK OF PHOTOGRAPHY 



objective has greater light-gathering power than a dry objective. This light-gathering 

 power or numerical aperture supplies a measure for several essential qualities of an 

 objective. Alibe, who first defined the conditions, expressed the relationship by the 

 formula 



N.A. = n sin U (2) 



where n is the refractive index of the medium contained between the specimen and the 

 front lens of the objective and U the semiapertural angle of the system. 



Front /ens of objective p^ ^ 



Fig. 2. — Optical conditions prevailing in metallographic microscope. 



FRONT LENS OF AN OIL 

 IMMERSION OBJECTIVE 



FRONT LENS OF A 

 DRY OBJECTIVE 



COVER GLASS — "^ 



AIR ^ 

 GLASS 

 MICROSCOPE 



SLIDE 



AIR 



>^ 



WATER 

 OR OIL 



TRONT LENS OF THE CONDENSER' 

 Fig. 3. — Comparison of paths of light rays for oil immersion and dry objectives. 



The resolving power of the objective is directly proportional to the numerical 

 aperture, and the brightness of the image to the square of the numerical aperture. As 

 the numerical aperture increases, the depth of penetration, i.e., the power of the 

 objective to resolve detail simultaneously at different depths or distances from the 

 objective, and the flatness of the field both decrease. 



