CHAPTER V 



CONDENSERS 



THE optical part of the illuminating system consists 

 of one or more lenses, used, as their name implies, to 

 condense the light from the radiant, and illuminate a 

 small area very intensely. The complete system is made 

 up of two separate units, " the substage condenser " and 

 the " collecting lenses." The latter are also known as 

 "auxiliary condensers," " parallelisers," or "bench 

 condensers," denoting that their function is to collect 

 the light from the radiant, and project it on to the sub- 

 stage condenser, or that they are used to project a parallel 

 beam of light, and that they form part of the optical 

 bench rather than of the microscope. The function of 

 the substage condenser is the illumination of the object 

 by a cone of light whose angle is comparable with that of 

 the objective in use. In this chapter the types of lenses 

 available for these purposes are described ; a discussion 

 of the manner in which they are used being left over to 

 succeeding sections. 



Diaphragms and light filters are also necessary items 

 in the illuminating system, as strict control over the 

 amount of light, the angle of the illuminating cone, the 

 illuminated area, and the constitution of the light, are 

 essential to success in photomicrography. 



Aplanatic Cone of a Condenser. The simplest con- 

 densers are uncorrected double-convex and plano-convex 

 lenses, or partially corrected combinations of two or three 

 lenses, but there are others in which the spherical and 



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