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OSCAR W. RICHARDS 



by the lowest refractive index (n) of the medium with which it is 

 used, e.g., air, water, or oil of the same index as the front lens of 

 the objective (Fig. 2). The resolving power of an objective is 

 X/(2n sin U), or X/(2 N.A.), when its aperture is filled with Hght by a 

 condenser. The resolving power for white light is usually calculated 

 with X = 555 m/x, the region to which the eye is most sensitive. 

 This formula gives the Abbe limit of resolution and modern research 

 indicates that resolution may be better than this value, perhaps 80% 

 of it. Note that this value is for the separation of two points. 



With an objective of N.A. = 0.85, the preceding formula gives a 

 limit of resolution of 0.47 ju for infrared radiation of 800 m/x, of 0.33 n 

 for light at the maximum sensitivity of the eye at 555 m^t, and 0.2 



DRY OBJECTIVE 

 N.A. = sin 6/ 



(/7=1) 



ton U^ab/d 



IMMERSION OBJECTIVE 

 N. A.= /72 sin U^ 

 - n^ sin 6^3 

 tan ^j= cf/d 



Level of 

 focus 



Level, 

 of scale 



Oil (/?2) 



Fig. 2. Numerical aperture and its measurement. 



y. for ultraviolet of 257 m^i wavelength. With N.A. = 1.4 the limit 

 for 555 m/x light is 0.2 /x. The greatest numerical aperture available 

 is 1.6, but its use requires not only slides, but also mounting and im- 

 mersion media of correspondingly higher index than ordinary crown 

 glass; it has not proved generally useful. Such a high aperture lens 

 also has a shorter working distance. When the approximate size of 

 the detail to be examined is known, the above limiting values can be 

 an aid in selecting the objective to be used and in the decision whether 

 to use a light or an electron microscope. 



While resolution is important and necessary, it is not sufficient 

 for observation. The image must be bright enough to stimulate the 

 recording medium, adequate contrast must exist, and magnification 

 must be great enough to match the resolving power of the observer. 

 For the eye a magnification of about one thousand times the numeri- 



