CHAPTER IX 

 THE CONDENSER 



Diiferent Kinds of Condensers. — Ordinary condensers 

 may be either dry or immersion; and each of these kinds 

 may be uncorrected (''Abbe" condenser), or corrected for 

 spherical aberrations only (aspheric, aplanatic), or corrected 

 for both spherical and chromatic aberrations (achromatic, 

 or achromatic-aplanatic) . In the best-corrected condensers 

 (achromatic-aplanatic), the sine law is so far observed 

 that the image of the source of light is fairly good somewhat 

 beyond the immediate neighborhood of the optic axis. 

 In the scientific use of the microscope, the source-field 

 (as previously defined) is, of course, changed with the 

 different objectives and eyepieces; and the source must 

 therefore be large enough, so that it is possible to make 

 its image on the slide equal to the object-field (as defined 

 in a previous chapter). 



Centering the Condenser. — There are at least four 

 ways of centering a condenser provided with centering 

 screws or an excentric centering ring; or of testing the 

 centering of a condenser unprovided with such devices, 

 (a) The center of the top lens may be marked accurately 

 with an ink dot, and the condenser centered by this. 

 This may be done by making a disc of card by the aid of 

 compasses, the size of the mounting of the top lens, and 

 perforating its center. (6) If the iris diaphragm is (as is 

 best) attached to the condenser, and is, as usual, somewhat 

 below the anterior focal plane of the condenser, the image 

 of the contracted iris, as seen with the low objective, may 

 be used for centering. This is the common method, 

 (c) If the back of the objective is observed through a small 

 centered opening at the top of the microscope tube, such 

 as a card disc with a rim and a small central hole, the light 



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