Polarization in Biaxial Crystals. 369 



manner in a crystal ; but in solution we have to deal with 

 the average effect for all possible directions. This average 

 will naturally be much less than the maximum value. 



If the plate of sugar is examined by circularly polarized 

 light, a single spiral is seen, resembling one of the two seen 

 in the corresponding experiment with quartz. With a plate 

 •17 inch thick, cut perpendicularly to the axis with the stronger 

 rotatory power, half a turn of the spiral was seen, resembling 

 the written letter c. With a thicker crystal a complete turn 

 was seen. The spiral can be seen equally well in white or in 

 monochromatic light. 



The method with a mirror, often used for the obser- 

 vation of Airy's spirals in quartz, is unavailable without 

 modification, for, since the sugar crystal is not symmetrical 

 about an optic axis, the two rays, before and after reflexion, 

 must pass in the same direction through the plate, not merely 

 in directions equally inclined to the axis which is under 

 observation. This difficulty can be overcome and the spiral 

 observed by the following arrangement of apparatus, the 

 pieces being named in order from above downwards : — Nicol 

 prism with its axis vertical, glass plate inclined to the hori- 

 zontal at the polarizing angle, plate of sugar, lens of about 

 1 inch focus, horizontal glass mirror with its plane accurately 

 passing through the principal focus of the lens. The plate of 

 sugar should be approximately at the other principal focus of 

 the lens, and the inclined glass plate and the nicol should be 

 as close to the plate of sugar as possible. The course of the 

 light is the same as in the Norremberg doubler. The spiral 

 was seen with perfect distinctness when a plate *17 inch thick 

 was used. With a thicker plate the spiral was hardly better 

 seen, the decrease in apparent size due to the increased thick- 

 ness apparently counterbalancing the improvement due to the 

 increased rotation. The phenomenon was equally clear in white 

 and in monochromatic (sodium) light. The spiral could only 

 be seen inside the first ring, and was like a perverted S. 

 Faint brushes commenced at the first ring. 



This method of observation can with advantage be used 

 with quartz also. The lens mentioned above must be replaced 

 by one of shorter focus, or by a converging system of lenses, 

 and a new lens (or system) must be placed over the quartz. 

 The quartz can then be held in the hand and inclined as 

 desired in order to see the various parts of the system of 

 spirals to the best advantage. 



Sugar crystals are liable to be irregularly crystallized ; in 

 the above experiments crystals that did not show the rings as 

 complete circles in monochromatic light were rejected. 



