54 THE MICROSCOPE 



diaphragm (Fig. 46) which enters into the construc- 

 tion of the substage condenser. 



7. The substage condenser is a necessary part of the 

 optical outfit. Its purpose is to collect the beam of 

 parallel rays of light reflected by the plane mirror, by 

 virtue of a short focus system of lenses, into a cone of 

 large aperture (reducible at will by means of an iris 

 diaphragm mounted as a part of the condenser), 

 which can be accurately focussed on the plane of the 

 object. This focussing must be performed anew for 

 each object, on account of the variation in the thick- 

 ness of the slides. 



The form in most general use is that known as the 

 Abbe (Fig. 47) and consists of a plano-convex lens 

 mounted above a biconvex lens. This combination 

 is carried in a screw-centering holder known as the 

 substage below the stage of the microscope (Fig. 40, /) , 

 and must be accurately adjusted 

 so that its optical axis coincides 

 with that of the objective. Ver- 

 tical movement of the entire 

 substage apparatus effected by 

 means of a rack and pinion is a 



' IG - 47 -l?uSt P of fAbMdecided advantage, and some 

 means should be provided for 



temporarily removing the condenser from the optical 

 axis of the microscope. 



With the oil immersion objective, however, an 

 achromatic condenser, giving an illuminating cone of 

 about 0.9, should be used if the full value of the lens 

 is to be obtained. It is generally assumed that a good 

 objective requires an illuminating cone equivalent to 

 two-thirds of its numerical aperture. The best Abbe 

 condenser transmits a cone of about .45 whilst the 

 aperture of the T V inch immersion lenses of different 

 makers varies from i.o to 1.4, hence, the efficiency of 

 these lenses is much curtailed if the condenser is merely 



