62 PROCEEDINGS OP THE AMERICAN ACADEMY 



position I have mentioned, so that the planes of the optical axes should 

 be inclined at an an^le of 60*. The result was — when the thickness 

 of the plates in each position were nearly equal — that a symmetrical 

 ring system was obtained, in which the optical angle was about IS*-*, — 

 the smallest I had measured ; and, by varying the relative thickness, 

 intermediate degrees of optical divergence were produced. By now 

 introducing laminte into the comjjound crystal, in the position of the 

 third member of the 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 uniaxial. Although these experiments were suffi- 

 cient to show that the macling was an adequate explanation of the 

 apparent variation 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 color 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 hyperbolas proportionally 

 indefinite. I therefore next made a similar experiment with a well- 

 known phlogopite mica from Jefferson County, N. Y.. whose crystals ai'e 

 very distinctly macled after the type of Fig. 3 or 5, the plates presenting 

 a variation of optical angle similar to that I have described, the normal 

 axial divergence being about ISJ*^. A very clear portion of one of 

 these plates was first cut into a regular hexagon, one of whose diagonals 

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

 split into twelve laminae, which were superimposed with the interven- 

 tion of balsam, and in alternating positions, like the members of a 

 made, — the optical plane in each of the lamina? making an angle of 

 GO'* with that of the lamina above or beneath it. The result was an 

 essentially uniaxial plate, differing from a plate of uniaxial mica only 

 in small irregularities in the contour of the rings, such as the lamination 

 would be expected 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 presenting 

 optical phenomena similar to those of a plate of quartz cut perpen- 

 dicularly to the principal axis. At the first trial I obtained a compound 

 mica plate showing a disk of color in the centre of the field whose tint 

 changed during the rotation of the analyzer, — of the polarizing micro- 

 scope, — like a plate of left-handed quartz ; and, on superposing a jilate 

 of right-handed quartz, the spirals of Airy at once appeared. The 

 rings, however, were wholly broken up, and appeared only in irregular 



