188 F. E. Wright — Transmission of Light through 



the error incurred thereby being of a comparatively low order 

 of magnitude and usually not over 1°. 



Uji i axial Crystals. 



Of the uniaxial crystals, calcite and nephelite were chosen 

 for measurement ; apatite plates were also tried, but they 

 proved to be inhomogeneous and, therefore, unsuitable for 

 accurate work. 



Calcite. 



On cleavage plates of calcite several different sets of observa- 

 tions were made : (1) The uniradial azimuths of monochro- 

 matic light waves emerging from bare cleavage surfaces were 

 determined for different positions of the crystal plate. (2) 

 For these same positions of the plate the points of minimum 

 illumination (maximum extinction) under crossed nicols were 

 ascertained. (3) The same cleavage plate was then mounted 

 in Canada balsam (?i Na = 1*537) between object glass (?i Na = 

 1*511) and cover slip (?i Na = 1*520), and the positions of mini- 

 mum illumination were determined between crossed nicols, 

 both with and without the use of the bi-quartz wedge plate. 

 (4) These observational data were compared with the values 

 obtained by calculation from the formulas (41), (42), (43), (44), 

 (37) of the preceding section for the same positions of the 

 crystal plate. (5) Although not strictly comparable with the 

 other angles, the azimuths of the radius OD (fig. 12) for both 

 the ordinary and extraordinary waves were determined graph- 

 ically. 



In Table II, the results of these observations and calcula- 

 tions are assembled ; in this table, * = angle of incidence, or, 

 more correctly, the angle of tilting of the crystal plate ; r = 

 angle of refraction (horizontal line No. 1 of the table) ;' m = 

 azimuth of the section z z' with the plane of incidence (head 

 line of table) ; 8 = azimuth of plane of polarization either of 

 extraordinary wave (E) or of ordinary wave (O) (horizontal 

 line No. 2) ; e u = uniradial azimuth of entering wave (hori- 

 zontal line No. 3) ; 8' = azimuth of emergent wave correspond- 

 ing to refractive wave 8 (horizontal line No. 4). 



Uniradial azimuths of emergent waves. — On the horizontal 

 line No. 5 the observed azimuths of the emergent waves O and 

 E are listed. The observations were made in strong sodium 

 light and each azimuth was determined ten times. In the 

 table each angle listed is the average of 40 readings, 10 for the 

 position of the plate +i, + &>, (5 of each set of 10 readings 

 being made on turning the stage clockwise, and 5, counter- 



1 In these calculations, the values u — 1-6585, e — l - 4864, and = 44° 37' 

 were adopted. 



