CHAPTER II 



Substage 

 condensers. 



ILLUMINATING APPARATUS AND SOURCES 

 OF ILLUMINATION 



A MOST important part of the microscope has now to be con- 

 sidered, namely, the substage condenser, which is essential with 

 all higher powers to converge a beam of light upon the object 

 in order to illuminate it brilliantly and to vary the character 

 of the illumination. 



There are three difierent kinds of substage condensers. The 

 simple so-called Abbe condenser consists of two lenses with an 

 iris diaphragm close behind the back lens and a tray below for 

 the insertion of patch-stops or colour filters, as shown in Fig. 13. 

 It was in use under various names long before the time of Abbe, 

 who, however, popularised it in a particular form of mounting. 

 It does not focus the rays correctly to one spot (see Fig. 14), 



— 



Fia. 13.— No. 3286, Abbe Condenser. 



Fig. 14. 



the oblique rays coming to a nearer focus than the more direct, 

 and does not form a definite image of the source of illumination 

 due to the uncorrected lenses of which it is constructed. It 

 has an aperture of 1 N.A. — that is to say, it will give 180° in air, 

 the maximum aperture obtainable with a dry condenser. A large 

 beam of light from the mirror thrown upon the back lens is 

 concentrated upon a small area (0). This area is illuminated by 

 an imperfect image of the source of light. The condenser fits into 

 the substage of the microscope by which it can be moved up and 

 down, or "focussed," or can be moved laterally, or "centred," until 

 the illuminated area (0) coincides with the object being examined. 

 The object is by this means brilliantly iUuminated. A powerful 

 illumination is often required to overcome the loss of light due 



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