DEPTH OF FOCUS 319 



TABLE VIII-1 Cont. 



Magnification Table 



Apochkomatic Objectives — Compensating Eyepieces 

 Magnifications and Real Fields 



Microscope tube length 160 mm. linage distance 250 mm. Real fields (italics) in mm. 



By Courtesy of Bausch & Lomb Optical Company, Rochester, N. Y 



exceed 180° of light in air. By referring to Fig. VIII-4 it becomes 

 apparent that a given lens has greater light-gathering power when 

 immersed in oil than when immersed in air. 



Since the brightness of an image is proportional to the square of the 

 numerical aperture, there is a definite additional advantage in the use 

 of an oil-immersion objective. 



Depth of Focus 



With objectives of low numerical aperture it is possible to change the 

 focus markedly up or down without loss in sharpness of detail. In 

 objectives of large numerical aperture a sharp focus is almost imme- 

 diately lost for a slight shift in vertical motion of the objective. It will 

 be noticed by referring to Fig. VIII-5 that the same slight shift above 

 or below the focal point produces very large circles of diffusion in the 

 large-numerical-aperture objective but only small circles of diffusion 

 in the low-numerical-aperture one. Thus the appearance is only 



