Transparent Inactive Crystal Plates. 197 



Aragonite. — The polished plate used was nearly normal to 

 the acute bisectrix. The observations were made in sodium 

 light with the aid of the bi-quartz wedge plate, and the usual 

 microscope lens system without condensor. The bare plate 

 was mounted as usual on the universal stage and the positions 

 of extinction for each position of the plate determined 12 

 times. The individual readings thus obtained agreed fairly 



TABLE V. 



i u a b Diff. 



50° 00' 90° 00' 87° 36' 3° 18' 90° 54' 



00 93 19 1 01 92 18 



35 00 



90 00 



86 59 



5 07 



92 06 



a 



75 00 



84 34 



7 26 



92 00 



a 



60 00 



83 03 



9 18 



92 21 



a 



45 00 



82 16 



7 16 



89 32 



a 



30 00 



81 15 



7 57 



89 12 



a 



15 00 



87 36 



2 24 



90 00 



it 



00 



94 57 



07 

 14 



95 04 



30 00 



!< 



86 53 



87 07 



25 00 



it 



95 38 



1 30 



94 08 



40 00 



U 



87 07 



07 



87 00 



19 30 " Optic axis A. 



12 00 " Optic axis B. 



well, the upper and lower limits of each set of 12 readings 

 being usually about 30' apart, but a high degree of accuracy 

 cannot be claimed for the angles listed in Table V, the prob- 

 able error being possibly 15'. The different positions of ex- 

 tinction thus observed are not precisely 90° apart, and often 

 differ from it by several degrees, again emphasizing the diffi- 

 culty of an accurate determination of the plane of polarization 

 of a light wave transmitted obliquely through a crystal plate. 



To find the direction of vibration for a dark point H on an 

 interference figure. 



An interference figure is obtained by passing a cone of con- 

 vergent polarized light waves through a crystal plate and 

 observing tbe interference phenomena as they appear in the 

 rear focal plane of an objective of short focus when examined 

 through an analyzer. In the course of their passage through 

 the microscope, the light waves emerging from the lower nicol 

 (polarizer) may be considered practically parallel, plane polar- 

 ized waves. On transmission through the condensor lens sys- 

 tem, their directions of propagation are changed and they 

 emerge from it in a sharply convergent cone ; but their line of 

 vibration has remained in the same plane except for the slight 



