210 F. E. Wright — Transmission of Light through 



large extent and there is no position of total extinction for the 

 tilted crystal plate even if the upper nicol be rotated alone. 

 In general it may be stated that from an incident plane polar- 

 ized wave two refracted waves are formed, which on emerg- 

 ence from the plate are each still plane polarized, but their 

 planes of polarization are not precisely 90° apart. The result- 

 ant light as observed through the analyzer is consequently 

 elliptically polarized and there is no possible position of total 

 extinction of the plate, but rather a region of minimum illumi- 

 nation which may extend over several degrees. 



These relations have an important bearing on methods based 

 on the determination of the positions of extinction of obliquely 

 transmitted waves, and preclude at once a high order of accu- 

 racy in the measurements. If the observed crystal plates are 

 mounted in Canada balsam, the rotatory influence of the sur- 

 faces of the glass and Canada balsam mount enter the problem 

 and tend to complicate the phenomena still further. 



The measurements of Fart 2 show : (1) That a tilted glass 

 plate may rotate the plane polarization of a transmitted plane 

 polarized light wave several degrees, and that the amount of 

 rotation increases with the angle of tilting; (2) that the 

 observed uniradial azimuths of tilted cleavage plates of calcite 

 agree closely with the calculated values ; (3) that for the central 

 areas of tilted plates of calcite, nephelite, muscovite, and ara- 

 gonite, there are no positions of total extinction. If settings 

 be made at the apparently darkest positions of the plate during 

 the rotation of the microscope stage, these positions are often 

 several degrees from 90° apart, and if the observed azimuths 

 of the plane of polarization be taken as the azimuth of the 

 refracted waves within the crystal, errors of several degrees 

 are easily possible. (4) An obliquely transmitted wave will 

 be extinguished provided its direction of vibration after emerg- 

 ence is contained in the extinguishing plane of the analyzer. 

 The direction of vibration of an observed dark point on the 

 axial bar of an interference figure is therefore the line of 

 intersection of the extinguishing plane of the upper nicol with 

 the polar plane of the given point. This construction, sug- 

 gested by the writer, does not take into consideration the rotatory 

 effects of the surfaces of crystal plate and glass mount, and is 

 accordingly only an approximate method. Prof. Becke has 

 suggested another method, which is, in effect, to find the inter- 

 section of the polar plane with the great circle in stereo- 

 graphic projection, which is tangent to a line parallel with the 

 principal section of one of the nicols. The points obtained by 

 Prof. Becke's method are slightly different from those obtained 

 by the writer's method, but not sufficiently different to affect 

 the degree of approximation obtainable by such methods. In 



