220 APPENDIX. 



3. In consequence of which probably, they are seen to 

 exhibit no coloured rings, but only a cross. 



4. The cross is brightly coloured, red, green, blue, &c. 

 upon a white ground (the polarizers being supposed to 

 be parallel to each other). This has a beautiful appear- 

 ance, especially when several circles seen at once have 

 crosses of different colours. 



5. When the polarizers are placed at right angles, the 

 phenomenon complementary to the above is seen. For in- 

 stance, the circle which presented a red cross upon a white 

 ground now presents a black cross upon a green ground, 



6. In an intermediate position of the polarizers, the 

 circle just mentioned presents a red cross alternating 

 with a green one, thus dividing the circle into eight 

 sectors of coloured light. 



Other circles present other colours, but they all follow 

 the same analogy, and the crosses upon all the crystals 

 are in a parallel position. 



7. These crystals last longer than the former ones. 

 I have found some of these retain their structure for two 

 months ; the former kind seldom last in perfection more 

 than a day. 



8. It sometimes happens that their circumference is 

 imperfect, and presents a notched or jagged outline. 

 These have a very beautiful appearance, and have been 

 almost universally compared by those who have looked 

 at them to highly-coloured flowers with four petals ; the 

 cross upon them being so dark as to have the appearance 

 of being a division between the petals. 



