94 



Modern Microscopy 



with light, as in Fig. 27, under which circumstance the 

 condenser has an aplanatic cone exceeding the N.A. of the 

 objective. The aperture of the condenser can now be 

 limited by means of a diaphragm, and an approximate value 

 obtained for the size of diaphragm that is used. The edge 

 of this diaphragm should be so set that its edge is just seen 

 appearing at the margin of the objective as in Fig. 28. The 

 aperture of the condenser when used with this size of 

 diaphragm is therefore a shade less than N.A. '5 — say '45. 

 An objective having a larger N.A., say '95, is now employed, 

 and it will be found that the back lens of the objective is no 

 longer filled with light. Theoretically, this is the condition 

 under which the aplanatic aperture should be estimated, 

 but when a flat flame of a lamp is presented edgewise, its 



Fig. 27. 



Fig. 28. 



Fig. 29. 



Fig. 30. 



image has corresponding depth, and when one part is 

 focused on the object, other parts of the image of the flame 

 will necessarily be out of focus. There is therefore a 

 certain range of adjustment of the condenser within which 

 the effect (so far as it depends on focusing the light on the 

 object) will be pretty much the same. But these different 

 positions give different apertures to the condenser as 

 judged by the light reaching the back lens of the objective. 

 The condenser should then be gently racked upward until 

 the disc of light is at its largest (Fig. 29) ; until on a 

 further movement of the condenser two black spots appear, 

 one on either side of the middle of the disc (Fig. 30), which 

 increase as the condenser is further racked up. The last 

 point before the appearance of the black spots furnishes the 

 position in which the condenser has the largest aperture 



