196 F. F. Wright — Transit ixxion of Light through 



crossed nicols may not coincide precisely with the positions for 

 which the two sides of the inserted bi-quartz wedge plate 

 appear equally lighted : the angles listed in lines 5 and 6, 

 Table III, bear out this inference. In column 7, the positions of 

 the direction I) (fig. 12, p. 200) determined graphically, are 

 listed and will be discussed in a later section. From the above 

 observations it is evident that parallel, plane-polarized light 

 waves transmitted obliquely through a crystal plate do not 

 emerge as strictly plane polarized waves. There is, conse- 

 quently, no position of total extinction, but a region of mini- 

 mum illumination which may extend over several degrees and 

 which, of course, precludes accurate determinations. 



The two positions of approximate extinction, determined by 

 observation, may differ by a degree or more from 90°. The 

 unequal rotation of the planes of polarization by the glass sur- 

 faces of the mount is an important factor in this connection. 



For positions of the plate in which the direction of ths 

 transmitted light waves made a relatively acute angle with the 

 optic axis, the lack of parallelism of the transmitted waves was 

 keenly felt, and a position of even approximate extinction was 

 not attainable. In such iustances the determination was not 

 attempted. 



Biaxial Minerals. 



Of the biaxial minerals, muscovite and aragonite were 

 selected for measurement, but only a short set of readings was 

 made on each, the object being primarily to determine the 

 positions of maximum extinction as well as possible with the 

 bi-qnartz wedge plate. The observations were made in sodium 

 light and with the aid of the usual microscope lens system, 

 except that the condensor had been removed, 



Muscovite. — A fresh, bare, cleavage flake was used, and 12 

 readings were recorded for each position of the crystal plate. 

 The effects of elliptic polarization were not especially notice- 

 able except for the position i = 22° 30', a> = 22° 30'. Not- 

 withstanding this, the measured positions of extinction are not 

 in general 90° apart, but differ from 90° by several degrees in 

 some positions, as shown in Table IV. 



45° 00' 



22 30 





TABLE IV. 







u 



a 



b 



Diff. 



60° 00' 



10° 48 



78° 54' 



89°42' 



45 00 



14 14 



73 32 



87 46 



22 30 



15 44 



71 40 



87 24 



45 00 



32 41 



56 31 



89 12 



22 30 



54 46 



34 08 1 



88 54 



Elliptic polarization pronounced. 



