PLEOCHROISM. II 



PLEOCHROISM. 



Since the present research involves the examination of minerals 

 which are not isotropic, the question presents itself whether it would be 

 better to examine them in natural or polarized light. In the former 

 case we obtain all the absorption bands, irrespective of the direction of 

 transmission, but their observed intensities will not necessarily be real. 

 In the latter case, where the intensity of the transmitted energy depends 

 upon the direction of vibration of the polarized rays with respect to the 

 axes of the crystal (pleochroism), the observed intensities will be real, 

 the number and position of the absorption bands will depend upon the 

 direction of observation, but the accuracy is less the farther one pene- 

 trates the infra-red, due to loss in polarizing the source of energy. 



By these statements it is not intended to give the impression that 

 pleochroism is not worth considering. The work of Merritt 1 and of 

 Konigsberger 2 is especially valuable in demonstrating that absorption is 

 dependent upon the direction of vibration of the incident energy. The 

 visible spectrum is so narrow that it is difficult to separate the absorp- 

 tion bands, while, on the other hand, the infra-red is a vast and almost 

 unexplored region, and it is here that one would expect to resolve wide 

 absorption bands such as exist in the visible spectrum. The curves of 

 Merritt and of Konigsberger show this, especially the calcite curves, 

 which are entirely independent for the ordinary and extraordinary rays. 

 Nevertheless, in looking over the previous work, it seemed to the writer 

 that in the present state of our knowledge of absorption a greater 

 advance would be made by simply mapping the spectra of a great many 

 minerals for energy transmitted through them in its natural mode of 

 vibration. 



The results have been most gratifying. If we had been content with 

 the examination of a few, easily obtainable, large-sized crystals like 

 colemanite, for pleochroism, the results would have been quite different. 



The maxima of the absorption of water are constant in position for 

 the amorphous and the crystalline minerals studied, which would indi- 

 cate that in this case the transmission is not affected by the direction 

 of vibration. 



The desirability of examining many minerals is illustrated in the 

 appended curves, in which it will be seen that frequently in one region of 

 the spectrum the water bands are obliterated by an increase in the gen- 

 eral absorption or by an adjacent absorption band, while in another part 

 of the spectrum the water bands are very distinct. 



1 Merritt : Phys. Rev., 2, p. 424, 1894. 



2 Konigsberger : Ann. der Phys., 61, p. 687, 1897. 



