CH. II.] 



LIGHTING AND FOCUSING. 



43 



§ 79. Proper Numerical Aperture of the Condenser. — As stated 

 above, the aperture of the condenser should have a range by means of 

 properly selected diaphragms to meet the requirements of all objectives 

 from the lowest to those of the highest aperture. It is found in prac- 

 tice that for diatoms, etc., the best images are obtained when the object 

 is lighted with a cone which shall fill about three-fourths of the diameter 

 of the back lens of the objective with light, but for histological and 

 other preparations of lower refractive power only one-half or one-third 

 the aperture can be utilized. 



Fig. 44 Shows the image of the 

 flame (Fl.) in the center (C) of the 

 field of the microscope and illuminat- 

 ing the object. 



Fig. 45. Shows the image of the 

 fame (Fl.) at one side of the center 

 l Exc.) and not properly illuminating 

 the object. 



Fig. 44. 



Exc 



Fig. 45- 



To determine this in any case, focus the object carefully, take out the 

 ocular, look down the tube at the back lens. If less than three-fourths 

 of the back lens is lighted, increase the opening in the diaphragm — if 

 more than three-fourth's, diminish it. For some objects it is advanta- 

 geous to use the entire aperture, for others, less than three-fourths. 

 Experience will teach the best lighting for special cases. 



Ob j 



Ob j 



Ilium 



Ilium 



Fig. 46. 



Fig. 47- 



Figs. 46-47. Figures showing the dependence of the objective upon the illumi- 

 nating cone of the condenser. ( Nelson.) 



Fig. 46 (A). The illuminating cone f 10m the condenser (Ilium). This is 

 seen to be just sufficient to fill the objective (Obj.). 



(B). 7 he back lens of the objective entirely filled with light, showing that the 

 numerical aperture of the illuminator is equal to that of the objective. 



Fig. 47. (A). In this figure the illuminating cone from the condenser (Ilium.) 

 is seen to be insufficient to fill the objective (Obj.). 



(B). The back lens of the objective only partly filled with light, due to the re- 

 stricted aperture of the illuminator. 



