24 



BACTERIOLOGY 



ical aperture of the closing limb (n.a.) is the sine of half the angle 

 of the converging beam multiplied by the refractive index of the 

 medium (in this instance the vitreous humor). This is commonly 

 designated as n.a. The numerical aperture of the opening limb 

 of the beam (N.A.), proceeding from a point in the object to the 

 objective, is the sine of half the angle of this beam multiplied by 



Pig. II. — lUumi-. 

 nation by a hollow 

 cone of light converg- 

 ing upon the object 

 at a wide angle, by 

 use of the central spot 

 stop. Compare with 

 Fig. 14, and with Pig. 

 16. 



Fig. 12. — Illumination by 

 a broad beam converging upon 

 the object at a wide angle. 

 Only a few beams of parallel 

 rays from a distant point 

 source of light are represented 

 in the figure. Compare with 

 Pig. 17. 



Pig. 10. — Central illu- 

 mination by a narrow 

 beam. Three beams of 

 parallel rays, such as might 

 come from a large white 

 cloud, are represented. 

 Note that these rays reach 

 the object as almost verti- 

 cal rays, varying from the 

 vertical by only a narrow 

 angle. Compare with Fig. 

 IS- 



the refractive index of the medium through which it passes. 

 This is commonly designated as N.A. Many desirable properties 

 of objectives, other than magnification, such as brilliancy of 

 illumination, definition, and resolution in depth, also depend upon 

 the numerical aperture, which is therefore "perhaps the most 

 important single feature of objectives of high power. 



Another important optical part of the bacteriological micro- 



