Transparent Inactive Crystal Plates. 



201 



the equivalent point determined by the method of Professor 

 Becke. According to the writer's method of construction, the 

 directions of vibration of any dark point of the interference 

 figure, as viewed through the upper nicol, must lie in the 

 extinguishing plane of the upper nicol. The directions found 

 by Professor Becke's method are not in general contained in 

 this plane, and appear, therefore, to be incorrectly located. 

 Objection has been made by Professor Becke to the writer's 

 method because the lines T) and C (fig. 12) are not 90° apart 

 while the points F and G are precisely so. In answer to this it 

 may be stated that in any direction within a crystal plate, as H 

 in the uniaxial crystal plate of fig. 13, Z being the optic axis, 



two waves are possible whose directions of vibration D and C 

 (fig. 13) are strictly normal to each other and to the line of 

 propagation H. In the interference figure, however, these 

 directions are not observed along the line of propagation H 

 but as they appear in projection ; and in the plane of this pro- 

 jection the lines of vibration are not 90° apart. To assume, 

 therefore, that the planes of polarization of the two possible 

 waves as observed in the interference figure are tangent to 

 the two lines parallel with Y Z through H in stereographic 

 projection, obviously introduces an error. If the point appears 

 dark in the interference figure, its direction of vibration must 

 be contained in the extinguishing 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. 12), 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 



