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



In order that you may have the evidence on which my con- 

 clusions rest, I send several specimens : — 



1. A plate of magnesian mica (phlogopite) from Jefferson Co., 

 New York, with optical angle of about 14°. 



2. Made of the same, showing planes of optical axes inclined 

 to each other, on different parts of the plate, at an angle of 60°, 

 and also showing variation of optical angle in passing from one 

 part of the plate to the other. 



3. Compound plate essentially uniaxial, made by splitting 

 plate like No. 1 and piling up the laminae in alternating posi- 

 tions at angles of 60°. 



4. Plate of Muscovite mica with optical angle between 60° 

 and 70°. 



5. Compound plate showing left-handed circular polarization, 

 formed from last by piling thin laminse (^J-g- of an inch about) 

 in left-handed spirals. 



6. Compound plate showing right-handed circular polariza- 

 tion, formed from films of the same mica as 4 by piling in 

 right-handed spirals. By placing 6 on 5 (point marked a on a) 

 with the edges of the glass of 6 parallel with the pencil lines 

 on 5, you will see, in certain azimuths with the plane of polari- 

 zation, the Airy spirals beautifully reproduced. 



I am sorry that the last two specimens (by far the most inter- 

 esting) are not more perfect. Neither of them presents constant 

 features at different azimuths with the plane of polarization; 

 but No. 5 is more perfect than No. 6 ; and a comparison of the 

 two will show how I have approximated to the perfect result in 

 proportion as I have succeeded in making the laminse at the same 

 time sufficiently thin and of uniform thickness. Plates as good 

 as No. 5 can be readily prepared ; but to do better than this, 

 except by accident, involves selecting the laminse by actual mea- 

 surement with a spherometer ; and since the laminse must also 

 be taken from one and the same piece of mica and cut in parallel 

 positions, you can easily see that, even after succeeding in cleav- 

 ing the mica into sufficiently thin films, you may work for many 

 days before securing at least twelve laminse from the same film 

 of uniform thickness. These specimens, however, will illustrate 

 my paper, if not by what they are in themselves, at least by that 

 to which they point and closely approach. My observations 

 were made with a polarizing microscope designed by DesCloizeaux 

 and made by Soleil. The object is illuminated by a very strongly 

 converging beam of polarized light ; and the instrument brings 

 into the field at once the two hyperbolas of a plate of topaz with 

 an optical angle of over 80°. An instrument, however, which 

 will include in the field the two hyperbolas of the plate of Mus- 



