340 Dr B. Silliman, Junior's Optical Examination 



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 opportunity has been 

 found for prosecuting the investigation, while former exami- 

 nations have been revised. The results of the whole research, 

 as far as they are complete, are exhibited in the following 

 tables. 



Much yet remains to be done, not only in confirming and 

 extending 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 resemblance in leading physical properties have 

 hitherto been generally confounded under a common desig- 

 nation. This branch of the inquiry is far the most laborious, 

 requiring a large number of rigorous chemical analyses. A 

 beginning in it, however, has been made by Mr Craw of the 

 Yale Laboratory, who has completed three analyses of Phlo- 

 gopites from New York. The results of his research, which 

 are particularly interesting, will be found on a following page. 



The physical questions connected with the micas embrace 

 also the translucency of the several varieties in different 

 directions, the effects of heat and magnetism in varying the 

 angle of the optic axes, and the value of the latter under 

 monochromatic light in all parts of the spectrum ; and inves- 

 tigations on these points would well reward the observer.! 

 I had proposed the subject last-mentioned to my friend Mr 

 W. P. Blake, before my own observations were made, and he 

 has recently planned and constructed for himself an instru- 

 ment for observations and measurements of this sort. This 

 instrument appears to me particularly well adapted for this 



* Dana's Mineralogy, p. 690. 



t A few experiments were made by the author, aided by Mr W. P. Blake, 

 with Melloni's apparatus, to determine whether any relation in the transmission 

 of heat existed between various micas corresponding at all to the different values 

 of the optic axes. In these trials the mica plates were as nearly as possible of 

 the same thickness, and they were placed so that the normal was parallel to 

 the bundle of rays of heat. The instrument was so adjusted that the Locatelli 

 lamp deflected the needle in 10" of time 30° of the scale. Thus arranged, the 

 following results were obtained : — 



