CH. 77] 



LIGHTING AND FOCUSING 



45 



FIG. 44. Shows the image of the 

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

 field of the microscope and illuminat- 

 ing the object. 



FIG. 45. Shows the image of the 

 flame (Fl.} at one side of the centre 

 (Exc.} and not properly illuminating 

 the object. 



Exc 



FIG. 44. 



FIG. 45. * 



To determine this in any case focus upon some very transparent 

 object, 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-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 



O 



Obj 



Til 



Ilium 



FIG. 46. 



FIG. 47. 



FIGS. 46-47. Figures showing the dependence of the objective upon the ilium 

 inating cone of the condenser (Nelson}. 



FIG. 46 (A). The illuminating cone from the condenser {I Hum}. 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 cotidenser (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 



