Double Refraction of Quartz. 203 



between the directions (a) and (b), provided that one be nearer to 

 the axis, and the other further from it, than (A). Now it is well 

 known that the order of the rings depends on the number of 

 lengths of wave which the extraordinary ray is in advance or 

 retardation of the ordinary ray. At this place the advance or 

 retardation is suddenly altered by half a wave. Consequently the 

 order of the rings is suddenly altered by half an order : the rings 

 become faint, and then there is a saltus of half an order in the 

 colours. And the direction of the ray where this saltus takes 

 place being observed, it gives the direction in which the ordinary 

 ray has the same ellipticity as the incident light, which is known 

 from the position of the rhomb. 



By this reasoning I had satisfied myself that the relation be- 

 tween the direction of the ray and its ellipticity could be made 

 evident to the eye. On trying it with the apparatus just de- 

 scribed, I found that the appearance was exactly what I expected. 

 On placing the rhomb in position 315°, the spirals are perfect. 

 On turning it forwards, the internal folds break successively, (if 

 the crystal be left-handed) in a line nearly horizontal, and the 

 upper part of each unites itself with the lower part of the fold 

 next beyond it. This continues, the outer folds being broken 

 after the inner ones, till when the rhomb has reached 0° the ap- 

 pearance is that of perfect circles. On turning still forwards, the 

 successive circles break (beginning with the outer ones) in a line 

 nearly vertical : and when the rhomb has reached the position 

 45°, the spirals are perfect as before, but twisted 90°. Thus at 

 any position of the rhomb intermediate to 0°, 45°, &c. one or 

 more of the inner circles are complete, but distorted, and the ex- 

 terior circles are changed to two inwrapping spirals. It is plain 

 that the points where the spirals begin are the points where the 



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