Prof. J. P. Cooke on the Vermiculites. 259 



made (that is, with the plane of the optical axes in the position 

 of the third diagonal of the hexagon), the apparent angle could 

 be reduced still further, so that the plate was apparently uni- 

 axial. Although these experiments were sufficient to show that 

 the macling was an adequate explanation of the apparent varia- 

 tion of optical angle I had observed in the plates of ripidolite 

 and Jefferisite, they also raised the question how far the effect I 

 had obtained in my experiments might be due to the circum- 

 stance that, on account of the deep colour of these minerals, it 

 is only possible to experiment on very thin plates, with which, 

 of course, the rings of interference are very wide, and the hyper- 

 bolas proportionally indefinite. I therefore next made a similar 

 experiment with a well-known phlogopite mica from Jefferson 

 County, N. Y., whose crystals are very distinctly macled after the 

 type of figs. 8 or 10, the plates presenting a variation of optical 

 angle similar to that I have described, the normal axial diver- 

 gence being about 15^°. A very clear portion of one of these 

 plates was first cut into a regular hexagon, one of whose diago- 

 nals was in the plane of the optical axes. This hexagonal plate 

 was then split into twelve laminse, which were superimposed 

 with the intervention of balsam, and in alternating positions, 

 like the members of a made, — the optical plane in each of the 

 laminse making an angle of 60° with that of the lamina above 

 or beneath it. The result was an essentially uniaxial plate, dif- 

 fering from a plate of uniaxial mica only in small irregularities 

 in the contour of the rings, such as the lamination would be ex- 

 pected to produce. On repeating now this experiment with a 

 Muscovite mica having a wide optical angle about 63°, I obtained 

 a most remarkable and unexpected result — a structure present- 

 ing optical phenomena similar to those of a plate of quartz cut 

 perpendicularly to the principal axis. At the first trial I ob- 

 tained a compound mica plate showing a disk of colour in the 

 centre of the field whose tint changed during the rotation of the 

 analyzer (of the polarizing microscope), like a plate of left- 

 handed quartz; and on superposing a plate of right-handed 

 quartz the spirals of Airy at once appeared. The rings, how- 

 ever, were wholly broken up, and appeared only in irregular 

 patches of colour. This irregularity was due to the unequal 

 thickness of the laminse employed, and I found it impossible to 

 split up one and the same hexagonal plate of mica into laminae 

 which were sufficiently uniform for the purpose. But very satis- 

 factory 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, succeeded in cleaving off 

 very thin plates of considerable size and nearly uniform thick- 

 ness. Selecting one of these plates, I first divided it by means 



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