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CHAPTEE V 



THE SUBSTAGE AND AUXILIARY CONDENSERS AND OTHER MATTERS 

 OF IMPORTANCE FOR BOTH MEDIUM AND HIGH-POWER 



MICROGRAPHY. 



This chapter contains a full description of the following : 



(i) Substage Condensers and how to centre them. 



(ii) " Critical Light," and the " Critical Image," how to obtain it and its use. 



(iii) The auxiliary Condenser, and how to centre it. 



(iv) Monochromatic light — its use. 



(v) How the heat of the illuminant is kept from the microscope and the specimen. 



(vi) A description of a convenient form of exposure-shutter. 



(i) Substage Condensers and how to centre them. — The primary object 

 of the substage condenser is to collect light from the illuminant and concentrate it 

 upon the object. It consists of a system of lenses which form a cone of light, the 

 apex impinging on the specimen, and when thus used, without any stop, the object is 

 said to be illuminated by a " large solid cone of light.'' But if an iris or other dia- 

 phragm contract the aperture of the condenser, the base of the cone is reduced in 

 diameter, and the object is declared to be illuminated by a " narrow solid cone." If 

 a stop — say of a piece of cardboard, or preferably a piece of metal — be placed in the 

 base of the cone so as to prevent all rays passing through its centre, the object has 

 then a holloiv cone of light impinging upon it, and is said to be illuminated by 

 " annular light.'" 



Condensers, like objectives, are capable of being made of different foci as well as 

 of different numerical apertures, and also of being constructed chromatically, achro- 

 matically, or after the style of the apochromatic objective, when they are called 

 chromatic, achromatic, or apochromatic, respectively. The ordinary so-called Abbe 

 condenser is a chromatic one, and of no use in photo -micrography. It is a poor form 

 of condenser at the best of times, and from a scientific point of view we hav^ oft^u 



