ON LIGHT. 389 



formed of this mineral (unfortunately very rare), and a 

 bright point illuminated in succession with all the pris- 

 matic rays viewed through it, beginning with the red, 

 two images would at first be seen, the one formed by 

 ordinary refraction, fixed, the other gradually approach- 

 ing it ; at a certain stage of the illumination coinciding 

 with it ; then crossing to the other side and separating 

 more and more from it as the light verged more to the 

 extreme violet. The experiment, which would be a very 

 beautiful one, is recommended to the attention of those 

 in possession of such crystals which they may not be in- 

 disposed to sacrifice. 



(165.) Of the colours developed by circular polarization. 

 Quartz, or ordinary rock crystal is uniazal: and when 

 a plate of it of moderate thickness, cut from one of the 

 six-sided prisms in which it usually occurs at right angles 

 to its axis, is examined in the mode above described 

 with a polarizer and analyzing plate, a superb system 

 of coloured rings and black cross is exhibited but with 

 this peculiarity, that the cross does not come up to the 

 centre, and that the interior rings are blotted out and 

 obliterated by a round patch of coloured light; whose 

 tint, when the tourmalines are at right angles, varies with 

 the thickness of the plate ; being white when very thin, 

 and passing, for plates successively increasing in thick- 

 ness, through all the series of tints of Newton's trans- 

 mitted rings. Keeping to one plate, the tint also varies 

 on turning round the analyzing plate in its own plane, 

 and with this very extraordinary peculiarity, viz., that 

 while in some crystals a certain succession of colours is 



