Chemistry and Physics. 427 
similar to those of.a plate of quartz cut perpendicularly to the 
able mica from Grafton, N. Hs a few trials, succeeded 
mm cleaving off very thin plates of considerable size and nearly 
iform thickness lecting one o e plates, I first divided 
fection. "Thus, then, it a pears that, even with micas of the wides 
Optical angle, we can build up a structure which is optically 
uniaxial,* 
i i ield a dozen or more lami- 
kness, which are of sufficient size to yie 2 ch a film, by splitting 
n 
the 
P such as the succession of colors on revolving the anal, 
Spirals of Airy, yet the more delicate phases of the phenomena pag 4 gi ns 
~ changes color when revolved on its own plane, the ci 
and co and the violet cross disappears. Moreover, oe regards 
the conditions which determine the phase of the circular polarization, 
