372 Optical Examination of several American Micas. 
i ae 
‘en 
Arr. XXXVIL—Optical Examination of several American 
Micas ; by B. Sttumay, Jr., A.M., M.D., &c. 
Read before the American i for the Advancement of Science, at aay 
n, August, 1850 
Prior to the publication of the second edition of Dana’s Min- 
eralogy, little had been done in distinguishing the séveral species 
among American micas, and in allotting them to the various locali- 
ties. In connection with Professor Dana, the writer, during the 
liane of the Mineralogy through the press, made a number of 
observations respecting the optical properties of such micas: as 
Were at that time’ accessible. A summary of these observations 
will be found in that volume.* Since that work was published, 
the writer has continued and multiplied his observations as far as 
the whole research as far as they are complete are exhibited in 
the following tables 
uch yet remains 3 to be done, not only in confirming and ex- 
tending the present measurements and adding new ones from, 
unexamined localities, but still more in reference to the chemical |, — 
character of the several compounds, which from their great re- My j 
the Beant, of the several varieties in differeuit directions, 
the effects of heat and magnetism in varying the angle of the 
optic axes, and the value of “the latter under So hiokiaet light 
in all parts of the spectrum ;—and investigations on these points 
would well reward the observer.t 3) had proposed the subject 
Jast-mentioned, to my frieud, Mr. W. P. Blake, before my own 
observations were made, and he has recently ssalibed and con- 
structed for himself an instrument for observations and measure- 
—_— of this sort. This instrument appears to me particularly 
ne 
ae Mineralogy, p. 
ek A a experiments ag ee by the author, aided “PY Mr. W. P. —_ with 
M: apparatus, to determine whether ay ee relation e : 
x 1 Steen various micas 1S 0 ee at all to the diferent Bef ate optic 
_ @xes. In these trials the mica reas net the same of 
Jeteniaeel they wots placed v0 thot the ws at py = ‘bundle of roy® 
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