BACTERIOLOGY 



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. 



FIG. io. Central il- 

 lumination by a narrow 

 beam. Three beams of 

 parallel rays, such as 

 might come from a large 

 white cloud, are repre- 

 sented. Note that these 

 rays reach the object as 

 almost vertical rays, 

 varying from the vertical 

 by only a narrow angle. 

 Compare with Fig. 15. 



FIG. 1 1 . Illumination 

 by a hollow cone of light 

 converging upon the object 

 at a wide angle, by use of 

 the central spot stop. 

 Compare with Fig. 14, and 

 with Fig. 16. 



FIG. 1 2 . Illumina- 

 tion by a broad beam 

 converging upon the ob- 

 ject at a wide angle. 

 Only a few beams of par- 

 allel rays from a distant 

 point source of light are 

 represented in the figure. 

 Compare with Fig. 17. 



Another important optical part of the bacteriological micro- 

 scope is the substage illuminating apparatus, consisting of the 

 mirror, the iris diaphragm and the condenser. These are neces- 

 sary to illuminate minute objects so that they may be satis- 

 factorily studied at high magnifications. By the use of the iris 

 diaphragm and of the central spot stop, the ordinary condenser 

 may be made to furnish three different kinds of illumination, (i) 



