ON THE OPTICAL PHENOMENA OF CRYSTALS. 215 



revolving the plate of glass upon which they stand. This 

 beautiful appearance can be seen with a moderate mag- 

 nifying power. I measured the diameter of some of the 

 larger crystals, which I found to be from I -300th to 

 1 -600th of an inch. But there are many much smaller, 

 and indeed they may be seen decreasing in size, until 

 nothing remains visible of their structure but the four 

 luminous quadrants, appearing like four minute dots of 

 coloured light placed close together. 



I proceeded to examine the circles with a high magni- 

 fying power, and under favourable circumstances of illu- 

 mination, and I observed in them a very admirable 

 structure. 



Each circle has upon it one or more coloured rings 

 arranged concentrically, but the number as well as the 

 colour of these rings is different in different individuals. 



The innermost ring is deeply coloured or black, and 

 incloses a central space of white light, which is traversed 

 by the arms of the cross intersecting in the centre. This 

 part of the cross, which stands within the innermost ring, 

 is beautifully well defined, and perfectly black. The 

 general appearance resembles the figure 98, in Brewster's 

 Optics, which is a representation of the rings seen in 

 uniaxal crystals. It especially resembles it in the circum- 

 stance above mentioned, viz. the more defined outline 

 of the part of the cross which is within the innermost 

 ring. 



We have hitherto supposed the polarizers to be crossed; 

 but if we place them in a parallel position we shall see 



