SPECIFYING QUART/. CRYSTAL ORIENTATION 



249 



started along s po. It will see along e pi the ray that started along 5 pi'. 

 Every point on the lens will have associated with it a different direction 

 in the crystal. Therefore the eye will see a pattern like that of Fig. 2.29. 

 This is the principle of the conoscope. In the conoscope (the name means 

 "conical viewing") the source 5 is replaced by the image of a source, the 

 image being cast by a lens; see Fig. 2.31. Thus by the use of two similar 

 lenses we get twice as much working space as one lens would give. 



POLARIZER 



LENS 

 CRYSTAL 



Fig. 2.30 — The principle of the conoscope 



POLARIZER 



ANALYZER 



LENS LENS 



Fig. 2.31 — A practical conoscope 



Figure 2.32 shows a cross-section of the Western Electric conoscope. The 

 graduated dial shaft goes out through the bottom of the tank to give more 

 working room — older instruments had the shaft overhead and it was in the 

 way. The light source is a mercury arc lamp with filters to isolate the 

 5461A line. The lenses have a converging power corresponding to f: 0.6. 

 The focus is not changed by changes in the refraction of the oil — in fact, the 

 focus is the same with no liquid in the tank as when filled with liquid. This 

 is of some interest for those who might wish to use the instrument for 

 Rochelle salt and accordingly use a fluid of refractive index about 1.495 



