OF ARTS AND SCIENCES. 63 



patches of color. This irregularity was due to the unequal thickness 

 of the laiuinoe employed, and I found it impossible to split up one and 

 the same hexagonal plate of mica into laminte, which were sufficiently 

 uniform for the purpose. But very satisfactory results were obtained 

 in the following way. I selected for the purpose the very clear and 

 easily cleavable mica from Grafton, N. H. ; and, after a few trials, suc- 

 ceeded in cleaving off very thin plates of considerable size and nearly 

 uniform thickness. Selecting one of these plates, I first divided it by 

 means of a parallel ruler into strips, and from these strips cut out the 

 hexao-onal laminae by means of a steel pattern carefully made. The 

 thin mica can be cut with perfect accuracy by a sharp knife on a plate 

 of o-lass. The hexagonal laminas thus obtained, though coming from 

 different parts of the mica plate, were optically parallel to each other ; and 

 by drawing at the outset a line, with a sharp point, near the correspond- 

 ing edges of the several strips, tliis line served as a guide for placing the 

 hexagonal lamince. From laminae thus prepared, plates were made show- 

 in »• the familiar system of rings as perfectly as could be expected. The 

 best results were obtained with plates consisting of from twelve to twenty- 

 four laminiE ; and the character of the resulting plate, whether left or 

 rioht handed, was found to depend on the order of the spiral arrange- 

 ment. If in building up the pile the marked side of each successive 

 lamina is turned throun;h an angle of 60" in the direction of the motion of 

 the hands of a watch, the result corresponds to left-handed quartz if turned 

 in the reverse direction to right-handed, and on superposing two dis- 

 similar plates thus prepared I obtained again the spirals of Airy in great 

 perfection. Thus, then, it appears that, even with micas of the widest 

 optical angle, we can build up a structure which is optically uniaxial.* 



* The great difBculty in preparing these pLates is to obtain thin fihns of mica 

 of uniform thickness, whicli are of sufficient size to yield a dozen or more lami- 

 nas; and the more nearly we have succeeded in preparing such a film, by 

 splitting sheets of mica, the more closely we have been able to imitate the 

 phenomena seen under like conditions with a plate of quartz. We have been 

 able to work with films which measured with a spherometer only ^L of an inch 

 in thickness, and have not obtained good results witii those which were much 

 thicker, and when thinner than this tlie mica cannot readily be cut into sliape. 

 The least inequality in the thickness of the several laminae composing the same 

 plate more or less mars the effect ; and, although some of the striking features 

 seen with quartz may remain, such as the succession of colors on revolving the 

 analyzer, and even the spirals of Airy, yet the more delicate phases of the phe- 

 nomena disappear. The plate changes color when revolved in its own plane, 

 the rings lose their circular form and become confused, and tlie violet cross 

 disappears. ]SIoreover, as regards the conditions wliicli determine the phase of 

 the circular polarization, the law stated above can only be affirmed with cer- 



