402 WRIGHT— POLARIZED LIGHT IN THE 



lographers in the study of metal alloys. Unfortunately the explana- 

 tion of the phenomena presented by opaque bodies is somewhat in- 

 volved and the experimental measurement of the optical constants of 

 such substances is encumbered with difficulties which, in many cases, 

 allow only approximate results to be obtained at best. In view of 

 these facts it has seemed to the writer that a useful purpose may be 

 served by a critical discussion, based on the commonly accepted ver- 

 sion of the electromagnetic theory of light for absorbing bodies, not 

 only of the phenomena which might be of value for diagnostic pur- 

 poses, but also of the factors underlying the several methods, old 

 and new, which may be applied to the measurement of the few deter- 

 minable optical properties of opaque bodies. A proper appreciation 

 of the possibilities and also of the limitations of the application of 

 polarized light to the study of opaque substances can only be had by 

 a proper understanding of the fundamental principles involved and 

 of their relative significance in practical diagnosis. 



In the theoretical treatment of the general problem emphasis will 

 be placed on the phenomena resulting on perpendicular reflection 

 (angle of incidence = o), because most metallographic observations 

 with the microscope are made under conditions of vertical illumina- 

 tion. The attempt will be made to present the fundamentals of the 

 subject and to indicate the mode of derivation of the equations re- 

 quired. Although several new relations are given, the treatment as a 

 whole is necessarily along lines which have been followed by others. 

 In the preparation of the section on theory the following books and 

 articles have been specially consulted : P. Drude in Winkelmann's 

 " Handbuch der Physik," Vol. VI, and in Annalen der Physik ; 

 " Lehrbuch der Optik," by P. Drude ; " Lehrbuch der Kristall- 

 physik," by F. Pockels ; " Physical Optics," by A. Schuster ; " The 

 Analytical Theory of Light," by J. Walker ; and " Physical Optics," 

 by R. W. Wood. 



The results of the investigation may be summarized by stating 

 that in general the optical constants, such as refractive indices and 

 absorption indices, cannot be satisfactorily ascertained on small pol- 

 ished random sections ; that the application of polarized light en- 

 ables the observer ordinarily to determine whether the crystal plate 

 :- 'Isotropic or anisotropic and also to ascertain the degree of aniso- 



