98 Modern Microscopy 



upon the stage, there will be seen a central disc surrounded 

 by six diffraction spectra similar to Fig. 32. With the 

 f-cone of illumination, the surrounding spectra will, in 

 some cases, appear to overlap the central disc, and in 

 others will not appear to touch it. Our plan is to open the 

 diaphragm of the condenser only to such an extent that 

 the spectra just touch, but do not overlap, the central 

 disc. This would necessitate that in some instances we 

 should employ rather less than a f cone of illumination 

 and in others rather more than a f cone. We have not 

 been able to observe that any loss of resolution results from 

 this practice ; but, on the other hand, in our opinion detail 

 is more clearly seen, and appears crisper under these cir- 

 cumstances of illumination than any other. This system 

 is especially advantageous when monochromatic light is 

 used. 



It will be found of great advantage to become acquainted 

 with the appearance of the back lens of the objective when 

 working ; many hints of importance may be gleaned from it, 

 enabling manipulation to be effected with increased pre- 

 cision. For this purpose the ' dummy ' eyepiece referred to 

 on page 82 is a most useful adjunct. 



When working with monochromatic light, the condenser 

 must be focused so that the whole of the light which is 

 visible on the back lens of the objective when the eyepiece 

 is removed shall appear as nearly as possible of the same 

 colour. 



Condensers having a numerical aperture of 1*0 N.A. and 

 over require to be immersed in order that they may work 

 at their full aperture ; that is, a drop of immersion oil or 

 Canada balsam must be placed between the top lens of the 

 condenser and the object. It will be found generally that 

 the condenser is a little too long in focus for continuity 

 between the top lens of the condenser and the underside of 

 the object to be maintained. Under such circumstances an 

 additional thin 3x1 inch slip, or a piece of cover-glass, 

 should be placed under the object, which will enable the 



