62 PHOTOMICROGRAPHY 



2 N.A. 

 Limiting resolving power = 



A 



Equivalent to N.A.x 96,410 lines per inch, for white 

 light, A=-5269 M or 5269 A.U. (see p. 158 for definition 

 of units). 



An objective illuminated by a f " solid axial cone should 

 resolve 



N.A. x 70,000 lines per inch for white light. 



A normal eye can resolve lines separated by about 

 a-hr" a t 10"- Consequently, if a microscope magnifies 

 100 times, lines .T^OI/' apart ought to be resolved by 

 the objective, so that they may appear as ^IV apart 

 in the image. A 1" objective used with a x 10 eyepiece 

 gives 100 magnification, and, if full resolution is to be 

 obtained under these conditions, the objective should 

 be capable of resolving 25,000 lines to the inch. If 

 it is given a N.A. of 0-26, it will be able to do this with 

 an oblique beam of white light. 



Under the same conditions a J" objective should 

 magnify 400 times with the X 10 ocular, and be capable 

 of resolving 100,000 lines to the inch, requiring a N.A. 

 four times as great, or N.A. 1-04. That is, to obtain the 

 utmost resolving power appreciable by a normal eye, 

 an objective should be given a N.A. of 0-26 for every 

 100 magnifications it gives with a X 10 ocular, selected as 

 about the highest with which an achromat can perform 

 to advantage. 



The following table gives the limiting resolution 

 of objectives of various numerical apertures calculated 



2 N.A. 



from the formula - ' for orange, green, and violet 



A 



light, and also includes values for (N.A.) 2 , which measures 

 the brightness of the image, and is an important factor 



in the determination of exposures, and for , which 



N.A. 



