RESOLVING POWER 61 



Penetration. 

 Flatness of Field. 

 Working Distance. 



Some of these require fuller discussion, as they are of 

 great importance in photomicrography (see pp. 255-257). 



Resolving Power is the ability to display, distinctly 

 and separately, lines or dots which are very close to- 

 gether. This property is equally as important as magni- 

 fication, for without adequate resolution no increase 

 of magnification will make fine detail distinct. Detail 

 must first be resolved into its component parts, and 

 then amplified until the eye can see it well, and this 

 involves the necessity of giving high-power objectives 

 greater resolving power than low ones, so that their 

 increased magnification may be accompanied by the 

 ability to distinguish finer detail. 



Resolution is not only dependent on the N.A. of the 

 objective, but also increases as the wave length of the 

 light employed decreases, and this is equally true of the 

 finest apochromat as of an achromat. To achieve the 

 highest resolution of which either is capable in ordinary 

 photomicrography, monochromatic blue or violet light 

 is used ; but green often gives appreciably better results 

 with achromatic objectives, as it is nearer the wave 

 length with which the objective performs to the best 

 advantage. The greater ease with which focussing is 

 carried out in green over blue light must also not be 

 lost sight of. Resolution is impossible without a previous 

 elimination of the spherical and chromatic errors, and 

 this also is a most important function of monochromatic 

 light. 



The greatest number of lines to an inch, or millimetre, 

 that a lens can resolve by oblique light of given wave 

 length is twice the numerical aperture divided by the 

 wave length in fractions of an inch or millimetre : 



