RESOLVING POWER 317 



Everything else being equal, the resolving power is inversely propor- 

 tional to the wavelength of the visible light used to illuminate the details 

 of the object. Blue-light illumination produces a greater resolving 

 power than red light, under identical circumstances. For white light 

 the value of X is taken from the brightest part of the spectrum and 



o 



lies at wavelength 5560 A. 



A microscope objective having a 1.6 N.A. with the cone of illumination 

 filling the whole aperture will resolve two elementary lines on an object 

 separated by a distance of 5560 X 10~ 8 /2 X 1.6 = 1.74 X 10~ 5 cm, 

 a resolving power of about 6000 lines per millimeter. This resolving 

 power is the practical limit attainable using this wavelength. A 4-mra 

 0.85-N.A. objective will resolve lines separated by distances ranging 

 between 0.00062 and 0.00031 mm, depending upon the angular aperture 

 and wavelength employed. 



By angular aperture is meant the size of the cone of light which passes 

 from the object to the objective and becomes effective in producing the 

 image. 



In the eye the greatest acuity is attained for a pupilar diameter of 

 2 mm. The resolution obtainable with the eye, as we have seen, is 

 limited to a visual angle of 1 minute of arc. The resolving power is then 

 14 lines per millimeter. Its numerical aperture is therefore very small. 



In round numbers an objective of N.A. 1.00 will resolve 100,000 lines 

 per inch, and, since R.P. is proportional to N.A., a 0.25-N.A. will then 

 resolve 25,000 lines per inch and 1.30-N.A. will resolve about 130,000 

 lines per inch. If a structure is known to be of the order of 35,000 lines 

 per inch, for example, it is impossible to resolve that detail with an objec- 

 tive of N.A. 0.25, no matter how much magnification may be employed. 



An ideal objective of N.A. 1.00, used with white light (X = 5300A) 

 should resolve lines separated by 0.000265 mm. If these spaces are 

 magnified about 300 times they should become visible. That is, 300 

 times the numerical aperture suffices to disclose all details resolvable by 

 the corresponding objective, although many eyes would require as much 

 as 500 times the numerical aperture. 



Using 500 times the numerical aperture as required to resolve any 

 detail, so as to make the detail comfortably visible to the average eye, 

 what combination of eyepiece and objective is necessary to resolve 

 details in a specimen separated by 0.000075 cm, illuminated by white 

 light of average wavelength 5300 A? 



If N.A. 1.00 resolves 0.000265 mm, N.A. 0.35 will resolve 0.00075 mm. 

 Since we have stated above that a total magnification equal to 500 times 

 the numerical aperture is more than enough for the eye to resolve the 

 detail, the total magnification required for the specimen will be 165 X- 



