CH. II] LIGHTING AND FOCUSING 55 



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 a 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 microscope 

 and illuminator 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 arrange- 

 ments of the illuminator are deficient, it will be very nearly 

 centered. 



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

 mounting for this the most commonly used condenser whereby it 

 may be centered as described for the achromatic condenser instead 

 of by the crude methods described above. If the condenser mount- 

 ing regularly possessed centering screws as in the microscope of 

 Watson & Sons and there were 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 Lealand were selected as models the condensers need not 

 be so bulky, and would still retain all their efficiency. 



Fortunately the Royal Microscopical Society of London, which 

 has done so much toward standardizing microscopical apparatus, has 

 proposed a standard size for the substage fitting for the condenser 

 of 1.527 in. =38. 786 mm. (see § 53). 



§ 99. Mirror and Light for the Abbe Condenser. — It is 

 best to use light with parallel rays. The rays of daylight are prac- 

 tically parallel; it is best therefore to employ the plane mirror for 

 all but the lowest powers. If low powers are used the whole field 

 might not be illuminated with the plane mirror when the condenser 

 is close to the object ; furthermore, the image of the window frame, 

 objects outside the building, as trees, etc., would appear with un- 

 pleasant distinctness in the field of the microscope. To overcome 

 these defects one can lower the condenser and thus light the object 

 with a diverging cone of light, or use the concave mirror and attain 

 the same end when the condenser is close to the object (Fig. 54). 



