CH. II] 



LIGHTING AND FOCUSING 



45 



Fig. 44. Shows the image of the 

 flame {Ft.} in the center (C) of the 

 field of the microscope and illuminat- 

 ing the object. 



Fig. 45. Shows the image of the 

 flame [Ft. ) at one side of the centre 

 (Exc.) and not properly illuminating 

 the object. 



Fig. 44. 



Fig. 45- 



To determine this in any case focus upon some very transparent 

 object, takeout 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-fourths diminish it. For some 

 objects it is advantageous to use less than three-fourths of the aper- 

 ture. Experience will teach the best lighting for special cases. 



Obj 



Obj 



III u 



Ilium 



Fig. 46. 



Fig. 47. 



Figs. 46-47. Figures showing the depeyidence of the objective upon the ilium 

 inciting cone of the condenser {Nelson). 



Fig. 46(A). The illuminating cone from the condenser {Ilium). This is 

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



(B). The 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 conefrome 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 

 restricted aperture of the illuminator. 



§ 84. Aperture of the Illuminating Cone and the Field. — It 



is to be remarked that with a very small source of light the entire aper- 

 ture of the objective may be filled if a proper illuminator or condenser 

 is used. The aperture depends on the diaphragm used with the con- 

 denser. And the size of the diaphragm must be directly as the aper- 

 ture of the objective. That is, it is just the reverse of the rule for 

 diaphragms where no condenser is used (§ 67) ; for there the diaphragm 



