202 F. E. Wright — Transmission of Light through 



X' Z' at right angles to Y' Z' ; but with this direction the 

 present problem is not concerned, its object being solely to find 

 the direction of vibration of an observed dark point in their 

 interference figure, tbe extinguishing plane of tbe upper nicol 

 being given. 



In tbe last paragraph, one factor which lias profound influ- 

 ence on the phenomena actually observed, has been purposely 

 held in abeyance, and must now be considered in detail. Let P 

 be a direction on the axial bar in an interference figure along 

 which plane polarized light waves enter at uniradial azimuth. 

 At the upper and lower surfaces of the crystal plate the plane 

 of polarization of these waves suffers a slight rotation and as a 

 result the emergent waves no longer vibrate in their original 

 plane and are consequently not totally extinguished by the 

 upper nicol. The point P is not completely dark. Similarly, 

 let H be a point, adjacent to P on the axial bar of the inter- 

 ference figure, for which one of the emergent waves vibrates in 

 the extinguishing plane of the upper nicol. If this wave alone 

 were considered, the point H would appear completely dark, 

 but along H a second wave is possible whose plane of vibra- 

 tion after emergence neither coincides nor is at right angles 

 with the first ; it is not completely extinguished by the upper 

 nicol and accordingly illuminates H slightly. The two adja- 

 cent points P and H appear, therefore, only approximately 

 dark, and the narrow fringe between them is of about the 

 same degree of darkness. There is, in short, no point of abso- 

 lute extinction on the bar. The width of the bar increases as 

 the margin of the field is approached (fig. 15). As the posi- 

 tions of extinction of the two possible waves emerging in one 

 given direction in air do not coincide precisely, and are not 

 exactly 90° apart, there is evidently a range of weak illumina- 

 tion between the positions of uniradial total extinction. For 

 each of tbe two possible waves, however, the positions of extinc- 

 tion are precisely 90° apart, and if the crossed nicols be rotated 

 through 90°, there will be only a slight change, due to surface 

 film effects, in the position of the axial bars in the interference 

 figure from an unmounted plate. On mounted crystal plates 

 the rotary effects of the surfaces of the glass mount enter the 

 problem and there a rotation of the crossed nicols through 90° 

 often produces a small though perceptible shift in the position 

 of the axial bars, as is evident from the series of measurements 

 on interference figures, represented by (figs. 14a, b, I7a, b.) 



Observations in convergent %>olarized light. — Tbe measure- 

 ments were made in strong sodium light on clear mounted and 

 unmounted cleavage flakes of muscovite and anhydrite. The 

 petrographic microscope was first accurately adjusted, a cap 

 nicol being used whose vernier divisions read directly to 3', 



