368 F. E. Wright — Measurement of Extinction Angles. 



important defects which render their general application cum- 

 bersome and unpractical. 



In the foregoing pages, the intensity formulae for light trans- 

 mitted by crystalline plates under different conditions have 

 been developed and the attempt has been made to treat the 

 subject in such a way that the results attained shall be directly 

 applicable to the practical methods for determining extinction 

 angles under the microscope. In the following sections, the 

 different methods for accomplishing this end will be described, 

 with special reference to their general applicability and rela- 

 tive accuracy, and the conclusions reached in this mathematical 

 part will be used constantly as criteria of fundamental import- 

 ance. 



Methods. 



Extinction angles can be measured either in plane polarized 

 light or in convergent polarized light ; and in plane polarized 

 light the exact location of the positions of zero extinction is 

 fixed, either by observing relative intensities of monochromatic 

 light under special conditions or by means of the interference 

 colors resulting from the use of white light. In all measure- 

 ments of extinction angles it is imperative that careful attention 

 be given to the source of light, especially if monochromatic 

 light be used. The source should be as intense and steady and 

 uniform as possible in order that the variation in the source of 

 light itself be not mistaken for actual differences in the micro- 

 scopic field. The rays of light incident on the preparation 

 should, moreover, be as nearly parallel as it is possible to obtain 

 them. To meet these requirements satisfactorily requires both 

 time and patience, but in order to attain the best results they 

 cannot be overlooked. 



The microscope, moreover, should be in perfect adjustment, 

 the optical system should be accurately centered and the cross 

 hairs in the ocular should be precisely parallel witli the prin- 

 cipal planes of the crossed nicols. The adjustment of the 

 microscope is not a difficult task to accomplish if suitable 

 apparatus is at hand, and will be discussed briefly in part 2 of 

 this paper. 



Assuming the microscope to be in perfect adjustment and 

 the source of light satisfactory, we may use any one of the 

 following direct methods for measuring the extinction angle of 

 a particular crystal plate: 



Parallel Polarized Light. 



(1) The ordinary method, which consists in turning the crys- 

 tal plate under crossed nicols until the position of maximum 



