CH. II.} LIGHTING AND FOCUSING. 45 



§ 82. Arrangement of the Condenser. — The proper position of 

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

 traversing it is brought to a focus on the object. If parallel rays are 

 reflected from the plane mirror to it, they will be focused only a few 

 millimeters above the upper lens of the condenser ; consequently the 

 illuminator should be about on the level of the top of the stage and 

 therefore almost in contact with the lower surface of the slide. For 

 some purposes, when it is desirable to avoid the loss of light by reflec- 

 tion or refraction, a drop of water or homogeneous immersion fluid is 

 put between the slide and condenser, forming the so-called immersion 

 illuminator. This is necessary only with objectives of high power and 

 large aperture or for dark-ground illumination. 



§ 83. Centering the Condenser. — The illuminator should be cen- 

 tered to the optic axis of the microscope, that is the optic axis of the 

 condenser and of the microscope should coincide. Unfortunately there 

 is extreme difficulty in determining when the Abbe illuminator is cen- 

 tered. Centering is approximated as follows : Put a pin-hole diaphragm 

 over the end of the condenser (Fig. 48)— that is, a diaphragm with a 

 small central hole — the central opening should appear to be in the mid- 

 dle of the field of the microscope. If it does not, the condenser should 

 be moved from side to side by loosening the centering screws until it is 

 in the center of the field. In case no pin-hole diaphragm accompanies 

 the condenser, one may put a very small drop of ink, as from a pen- 

 point, on the center of the upper lens and look at it with the microscope 

 to see if it is in the center of the field. If it is not, the condenser should 

 be adjusted until it is. When the condenser is centered as nearly as 

 possible remove the pin-hole diaphragm or the spot of ink. The micro- 

 scope and illminator axes may not be entirely coincident even when the 

 center of the upper lens appears in the center of the field, as there may 

 be some lateral tilting of the condenser, but the above is the best the 

 ordinary worker can do, and unless the mechanical arrangements of the 

 illuminator are very deficient, it will be very nearly centered. 



It is to be hoped that the opticians will devise some kind of mount- 

 ing for this the most commonly used condenser whereby it may be cen- 

 tered as described for the achromatic condenser instead of by the crude 

 methods described above. If the condenser mounting regularly pos- 

 sessed centering screws as in the microscope of Watson & Sons (Fig. 

 71), and there was a centering diaphragm in the proper position so that 

 its image could be projected into the field of view, the operation would 

 be very simple. If, further, the condensers of Powell and Zealand 

 were selected as models the condensers need not be so bulky, and still 

 retain all their efficiency. 



