8 ELEMENTARY CHEMICAL MICROSCOPY 



objective to make clear objects or structures in more than one 

 plane. This is known as its penetrating power. The pene- 

 trating power of an objective has been shown to be inversely 

 proportional to the numerical aperture and to vary as the square 

 of the equivalent focus. 



Leaving out of consideration the numerical aperture, it is 

 found that the resolving power of an objective is inversely pro- 

 portional to the wave-length of light. By employing light rays 

 of very short wave-lengths we may thus obtain exceptional 

 resolution. 



In the consideration of numerical aperture it is usually assumed 

 that the illuminating cone of light completely fills the aperture of 

 the objective. Nelson l has shown that in practice with the older 

 types of objective we can rarely count upon more than three- 

 fourths of the available numerical aperture. Modern objec- 

 tives perform somewhat better. » 



In comparing objectives as to their ability to render struc- 

 tures clear and distinct it is usual to do so by computing the 

 number of ruled lines to the inch or millimeter each one will 

 make clearly visible (resolve). Since, as pointed out, we can- 

 not obtain the theoretical resolving power in practice a correc- 

 tion coefficient must be introduced into our formula. Nelson 

 assigns to this coefficient the value 1.3. The practical working 

 formulas then become: 2 



A V LI , ■ 2 N - A - 



Available resolving power = , 



Available illuminating power = I - : — '- 1 , 



\!-3 V 



. ., ,, . 1.3A 



Available penetrating power = ' . 



For white light a mean value may be assumed to be X = 5607 

 (= 0.5607 fi) and for blue light X = 4861 (= 0.4861 /x). 

 Advantage has been taken of the increased resolving power 



1 J. Roy. Micro. Soc, 1893, 15-17. 



2 J. Roy. Micro. Soc, 1906 521. 



