30 THE USE OF THE MICROSCOPE 



The average of the (diameters or radii of the) condenser cir- 

 cle and the objective circle measures the working aperture. 

 The condenser circle is filled with light from the source of 

 light. The dim ring formed by that part of the objective 

 circle which extends beyond the condenser circle (Fig. 9) 

 is lighted only, in the absence of glare, and with all 

 adjustments correct, by light diffracted, reflected, or 

 refracted from the object field. 



Fig. 9.^ — This shows, on the back of the objectives, condenser circles with 

 apertures from 0.25 to 1.10, with objectives from 0.3 to 1.3 in aperture. The 

 letter F shows the presence of "flooding" with light, due to the excess of the 

 condenser aperture over that of the objective. (The back lenses of the different 

 objectives are presumed to be equal.) 



A second chief rule of modern microscopy is that the 

 condenser circle should approach as near to equality with the 

 aperture circle of the objective as is possible without causing 

 glare. 



Above the eyelens of the eyepiece (at the eyepoint) 

 are the images thrown, by the eyepiece, of the objective 

 and condenser circles. These, of course, also consist of a 

 dim ring with a bright circle inside (and have been called 

 the Ramsden disc, the exit pupil of the microscope, etc.). 

 Here they will be called the eyepiece circles (objective 

 eyepiece circle, or condenser eyepiece circle). The eyepiece 

 circles should be sufficiently above the mounting of the 

 eyelens to be level with the pupil of the eye. The position 

 of the eyepiece circles (the eyepoint) needs, of course, to 



