400 C. U. Shepard—Mineralogical Notices. 
oe | 4a, 2a, 
gS As Gj or n,cos 1=n cos r=———-— 
Substituting, we have g— Teicie XS cosr 
2a 
= +n cos? r 
a , or finally, 
cos 7 cosr 
_2(m—a,)+n cos® f 
cos 7 cos 7 
(8) 
If the angle zis small, the value of n, will vary very little 
with 2, consequently there will be a large number of circles all 
nearly achromatised. Under favorable circumstances as many 
as one hundred rings have been counted, using an ordinary 
lamp, as source of light. The difference of path of the two 
pencils which produce these rings in white light may exceed a 
thousand wave lengths. 
Art. XLVII.— On two New Mi ated Monetite and Monite, 
with a notice of Pyroclasite; by CHARLES UPHAM SHEPARD 
with analyses, by C. U. SHEPARD, a 
THE specimens ay described were sent to us by Mr. John 
G. Miller of Ottawa, to whom Canadian ee owes so 
many inipeiieieat ' discoveries. “Dhay 4 e from the Twin 
islands, Mona and Moneta, wes are aidan forty miles from 
the port of Mayaguez, Porto 
1. Monetite.—The Moneta mineral, which we call monetite 
bird-guano. The soluble ae oe of this investment, in 
percolating the highly porous strata, have been thrown down 
in their transit in the me etamorphosed semen now met 
_unerystalline, more or less mpact , heterogeneous in their 
composition ; and contain coljsiderable traces of organic matter. 
They very rarely present dissociated simple minerals; 
polar ized, an ree from ongisie mpregnation as if 
derived from as or granite. The most wbendin Species in 
