50 ELEMENTARY CHEMICAL MICROSCOPY 



the other with an aqueous solution of nitrosodimethylaniline 



(i : 12000). 



The only changes in construction and materials lie entirely 

 in the illuminating devices. Any microscope permitting the 

 attachment of a dark-ground illuminator whose lenses are made 

 of quartz may be converted into a fluorescence instrument. 



Although this system of illumination is still so new as to have 

 been tried by but very few workers, its future development 

 seems assured and its usefulness in qualitative chemical analysis 

 of minute fragments of material to be unquestioned. 1 



It is valuable not only in the analysis of inorganic material, 

 such as crushed minerals, soils, mixtures of tiny crystals, etc., 

 but is of equal value in organic analysis, in the examination of 

 foods for adulteration and even in the microscopy of drinking 

 water. 



i. Polarized Light. — Few chemists realize the value of 

 employing polarized light in connection with the microscopic 

 examination of material to be analyzed, and few appear to 

 appreciate the great saving of time, labor and reagents that 

 such an application generally affords. Even a cursory exami- 

 nation with the most simple polarization devices is not to be 

 ignored. 



In the microscopic study of material of unknown composition, 

 the first step of the chemist should be to subject it to polarized 

 light. 



But in order that the polarization microscope may be employed 

 intelligently in the analysis of inorganic and organic materials, 

 it is essential that certain fundamental concepts of optics and of 

 crystallography be recalled; otherwise the phenomena observed 

 may not be properly interpreted. 



All transparent (and translucent) bodies behave with respect 

 to light waves in one of two ways: (1) they are optically homo- 

 geneous and therefore have no effect upon a beam of light sent 



1 See Heimstadt, Das Fluoreszenz-Mikroskop, Zeit. f. wiss. Mikros., 28 (191 1), 

 330; Wasicky, Das Fluoreszenz-Mikroskop in der Pharmakognosie, Pharm. Post, 

 (1913); Lehmann, H., Das Lumineszene-Mikroskop, seine Grundlagen und seine 

 Anwendungen, Zeit. f. wiss. Mikros., 30 (1913) 41 7- 



