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measurements under llic rnicroHCf^fM!. Care 

 should be exercised in e()in})iriiiif^ oculars 

 and objectives to juoid exc(!(;diiif5 useful 

 magnifications h)ased upon th(! previously 

 discussed criterion of numerical aperture. 

 This is especially true when measurements of 

 images are made. The halo around an over- 

 magnified image may cause the structure to 

 appear much larger than it is in reality. 



Condensers direct light to the specimen 

 and, along with the objective, determine the 

 numerical aperture of the microscope sys- 

 tems. Therefore, their numerical aperture 

 must be adjustable to that of the objective. 

 The condenser lens system should also be 

 corrected chromatically, spherically, and be 

 free of coma. Coma results when different 

 regions of a lens have different magnifica- 

 tions, creating spurious tailed images. A 

 condenser which has been corrected for all 

 three is referred to as an achromatic-aplan- 

 atic condenser. Ordmarih" the effective 

 numerical aperture of a condenser is subject 

 to alteration by means of an adjustable iris- 

 aperture diaphi-agm placed in an appropriate 

 position to define the angle of the cone of 

 light illuminatmg the specimen. This is im- 

 portant to reduce glare as well as to achieve 

 resolution. 



Alignment and Illumination. The use 

 of the condenser is so closely allied to meth- 

 ods of illumination and microscope ahgn- 

 ment that at this pomt it becomes desirable 

 to consider all three. In fact, condensers are 

 designated according to methods of illumina- 

 tion. There are both bright-field and dark- 

 field condensers, phase contrast, and hght- 

 polarizing condensers. Certam requirements 

 for alignment and illumination have been 

 expressed by several authors (1, 2, 9). One 

 of the best step-by-step charts available is 

 one published by Sliillaber (1). Briefly, the 

 centration of light som-ce and optical com- 

 ponents is accomplished, a field-limiting 

 aperture is imaged by the condenser onto 

 the sample plane and adjusted to coincide 

 with the field of view, and the aperture 



(condenser) diaphragm is used to control 

 glare. In general, these steps are suited to 

 bright-field aiiginnent in both transmitted 

 and \'('i-tical illumination. 



When a sample is illuminated, the light 

 nujdified by the sample creates the image. If 

 excessive amounts of undeviated light pass 

 through a specimen, inherently low in con- 

 trast, and enter the objective, the deviated 

 beam is unable to compete favorably and, 

 therefore, the image is comparati\'ely weak. 

 If the ratio of the intensities of the undevi- 

 ated to the deviated beams is reduced, the 

 image can be made relatively stronger. The 

 latter condition may be achieved by illumi- 

 nating the sample with a hollow cone of light, 

 whose inner angle is too great for undeviated 

 light to enter the objective lens; only the 

 fight widely scattered by the sample is seen 

 in the image. This is dark-field illumination. 

 Although highest possible numerical aper- 

 tures are employed in dark field, the image 

 is not as informative m some respects as 

 others observed under conditions of lower 

 resolution. This is because much of the devi- 

 ated light from the specimen is also pre- 

 vented from entering the objective. 



Dark-field condensers suited to both trans- 

 mitted and incident illumination are avail- 

 able. It is possible to employ varj-ing degrees 

 of hollow conical illumination approaching 

 dark field, and permitting some of the un- 

 de\'iated beam to enter the objective. More 

 often for expediency in obtaining contra.st, 

 the undeviated beam is reduced by adju.st- 

 ing the bright -field conden.ser diaphragm to 

 illuminate the specimen with a narrower 

 solid cone of light. Resolution suffers in the 

 latter method, and not m the former. Several 

 manufacturers make pha.se-contrast con- 

 densers which illuminate the sample with a 

 hollow cone suitable for semidark field, and 

 which may be u.sed effectively in producing 

 image contrast in conjunction with ordinary 

 objectives. Besides dark-field and bright- 

 field effects, there are special condensers for 

 optical staining, that is, they create different 



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