2G0 Prof. ,T. P. Cooke on the Vermiculites. 



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

 hexagonal laminse 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 glass. The hexagonal laminse thus obtained, 

 though coming from different parts of the mica plate, were opti- 

 cally parallel to each other; and by drawing at the outset aline 

 with a sharp point near the corresponding edges of the several 

 strips, this line served as a guide for placing the hexagonal 

 laminse. From laminae thus prepared, plates were made showing 

 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 laminse ; and the character of the resulting 

 plate, whether left- or right-handed, was found to depend on the 

 order of the spiral arrangement. If in building up the pile the 

 marked side of each successive lamina is turned through 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 

 dissimilar 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 difficulty in preparing these plates is to obtain thin films of 

 mica of uniform thickness which are of sufficient size to yield a dozen or 

 more laminae ; 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 4^-q of an inch in thickness, and have not obtained good results with 

 those which were much thicker ; and when thinner than this the mica cannot 

 readily be cut into shape. 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 colours on revolving the analyzer, and even the spirals 

 of Airy, yet the more delicate phases of the phenomena disappear. The 

 plate changes colour when revolved in its own plane, the rings lose their 

 circular form and become confused, and the violet cross disappears. More- 

 over, as regards the conditions which determine the phase of the circular 

 polarization, the law stated above can only be affirmed with certainty of 

 plates consisting of laminae which very nearly fulfil the conditions we have 

 described. Very small inequalities of thickness renders the effect irregular, 

 and made it at first difficult to discover the law. Our experiments have 

 been a series of approximations ; and although we may never be able with 

 our rude appliances to compete with nature in the manufacture of uniaxial 

 crystals, yet we have approached so near to the perfect result as to be able 

 to point out with confidence one way at least by which the effects seen in 

 natural crystals may be produced. We have usually cut the laminae into 

 regular hexagons ; but equilateral triangles might more easily be cut, and 

 would probably give as good results ; for although the errors of position 

 might not be so well distributed, we have found that a slight variation in 



