8 ELEMENTARY CHEMICAL MICROSCOPY 



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

 make clearly visible (resolve). To obtain these values the recip- 

 rocal of the above given standard formula must be taken. Since, 

 as pointed out, we cannot obtain the theoretical resolving power 

 in practice a correction coefficient must be introduced into our 

 formula. Nelson assigns to this coefficient the value 1.3. The 

 practical working formulas then become : 1 



Available resolving power = ' 



/N A \ 2 

 Available illuminating power = i - 1 ) > 



1.3 X 



/ 



Available penetrating power 



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

 (= 0.5607/1) and for blue light X = 4861 (= 0.4861 /*). 



Advantage has been taken of the increased resolving power 

 attainable by short wave lengths in the application of ultra- 

 violet light (X 25oo) to photomicrography. In this way a 

 resolving power of three times that obtainable with red light 

 (X 75oo) may theoretically be obtained. Since ordinary glass 

 is opaque to rays below X 3000, it is essential that the condenser, 

 objectives, oculars, object slides, etc., be made of quartz. For 

 similar reasons quartz is preferable to glass in all ultramicroscopy, 

 moreover, most glass exhibits a marked violet fluorescence under 

 the influence of ultraviolet rays; quartz does not. 



SELECTING OBJECTIVES. 



It is evident from the above briefly outlined considerations, 

 that the choice of an objective of a given equivalent focus and 

 magnification must depend upon the nature of the work the 

 objective will be required to perform. In microchemical analy- 

 sis, because of the rather unusual conditions which obtain, ob- 

 jectives must be selected with special reference to long working 

 distance and great depth of focus; the brightness of field and the 

 resolving power necessarily lost are, in this class of work, of 



1 Nelson, J. Roy. Micro. Soc., 1906, 521. 



