OPTIC AXIAL ANGLE. 
161 
projection, obviously introduces an error. If the point appears dark in the 
interference figure, its direction of vibration must be contained in the extin- 
guishing plane of the analyzer and it is with such points alone that the 
present problem has to do. Along the line of propagation OH* (Fig. 93) 
a second direction of vibration is possible at right angles to OD and normal 
to OH; this direction OC is in general not contained in the plane X'Z' at 
right angles to Y'Z' ; but with this direction the present problem is not con- 
cerned, its object being solely to find the direction of vibration of an observed 
dark point in the interference figure, the extinguishing plane of the upper 
nicol being given. 
In the last paragraph one factor, which has profound influence 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 
FIG. 950. 
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 // be a point, adjacent to P on the 
axial bar of the interference 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 vibration after emergence neither 
coincides with, nor is at right angles to the first; it is not completely extin- 
guished by the upper nicol and accordingly illuminates H slightly. The 
two adjacent points P and H appear, therefore, only approximately dark, 
*O being the center of the projection sphere. 
