Ch. II] 



CENTERING THE SUBSTAGE CONDENSER 



59 



N.A., the aplanatic cone transmitted is only 0.5, and it is the apilanatic 

 cone which is of real use in microscopic illumination where details 

 are to be studied. There is no doubt, however, that the results ob- 

 tained with a non-achromatic condenser like the Abbe are much more 

 satisfactory than with no condenser. The highest results cannot 

 be attained with it, however. 



§ 103. Position of the condenser. — The proper position of the 

 illuminator for high objectives is one in which the beam of light trav- 

 ersing it is brought to a focus on 

 the object. If parallel rays are re- 

 flected from the plane mirror to it, 

 they will be focused only a few milli- 

 meters above the upper lens of the 

 condenser; consequently the illumi- 

 nator should be about on the level 

 of the top of the stage and therefore 

 almost in contact with the lower sur- 

 face of the slide. 



§ 104. Determining whether the 

 condenser is centered. — For get- 

 ting the best results the condenser 

 should be centered to the optic axis 

 of the microscope; that is, the optic axis of the condenser and of 

 the microscope should be al,ong the same straight line. The simplest 

 method of determining this point with any given objective is to get 

 the microscope in focus on some very small or transparent object and 

 then to look at the eye-point above the ocular with a magnifier. While 

 looking at the eye-point close the iris diaphragm until the aperture 

 of the objective is only partly filled. Now, if the circle of light appear- 

 ing in the back lens is in the center, the condenser is centered. If 

 it is not in the middle, then the condenser is not centered (fig. 43). 



If the condenser has centering screws (fig. 40), it is very simple to 

 adjust them until the circle of light appears in the center of the back 

 lens. If centering screws are not provided and the condenser is 

 badly decentered, the microscope should be sent to the makers for 

 correction. 



Fig. 40. Achromatic Substage 

 Condenser with the Diaphragm 

 between the combinations. 



(From the Catalogue of Zeiss), 

 cs Centering screws. 



