PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 



775 



Figure 4 shows the relationship of theoretical resolving power, numerical aperture, 

 and dominant wavelength of light for immersion objectives having numerical apertures 

 ranging from 1.25 to 1.60. The equation expressing theoretical resolving power is 

 based on the assumption that the detail being resolved consists of equally spaced lines, 

 in other words, a very fine ruling or grating. 



The designation "lines-per-inch," as a measure for resolution, is not a very for- 

 tunate one because few specimens exhibit a uniform arrangement and spacing of detail. 

 Perhaps the nearest approach is in the age-hardening (or softening) phenomenon of 

 metals in which a hard constituent is precipitated from the matrix in a very fine par- 

 ticle size — so fine that we must see millions of particles only as a cloud. These par- 

 ticles under suitable treatment may be induced to coalesce and to grow in size so that 

 they may easily be recognized at low powers as single particles. The utmost in resolv- 

 ing power is required to reveal the smaller particles. 



■ N= Number of lines pen'nch 

 NA= Numerical aperture 



of lens 



Waveleng+h fn inches 



^220 



u 



^200 



w 



(D 



•-§ 180 

 <+- 



o 



w 



^ 160 

 c 



I 140 



■~ 120 

 .o 



-1- 



■| 100 

 w 



80 



4000 5000 6Q00 7000 



Dominant Wavelength in Angstrom Units 



Fig. 4. — Relationship of theoretical resolving power, numerical aperture, and dominant 



wavelength of light. 



Color Correction of Objectives. — The numerical aperture of an objective does not 

 disclose information concerning the chromatic or spherical corrections which have been 

 applied to the objective. The value of an objective also depends on the degree to 

 which aberrations inherent in a simple lens have been corrected. 



In the achromatic objectives the correction is least perfect of all, and in the apo- 

 chromatic objectives the correction is of the highest order. The semiapochromatic 

 objectives, as their name implies, occupy a position about intermediate. All lens 

 systems have some imperfections, in the fusion of the rays. 



The achromat is an objective which is designed for visual work. It is corrected to 

 work at its best with the particular color of light which is most effective to the eye, 

 viz., the yellow-green. This color is referred to as the preferred color. 



The achromats are corrected chromatically for two colors and spherically for one 

 color. As the extremes of the visible spectrum are approached, the fusion of the rays 

 becomes less and less complete. When an achromat objective is properly corrected, 

 residual colors of the secondary spectrum remain. 



Apochromatic objectives are corrected chromatically for three colors and spluM-i- 

 cally for two, and the fusion of the rays is more nearly perfect. The colors of tlic 

 secondary spectrum are eliminated altogether in a good objective, and only a faint 



