Geology and Mineralogy. 395 
ror M, is in one focus of the lens L, while ine image of S' oe 
cides with the mirror 8”, which is placed at the conjugate foc 
With this arrangement, when M turns alomty, the light from S” is 
reflected back through the lens, so that an image is formed — 
coincides with S. en, however, the mirror rotates rapidly, 
the position of M will — changed ‘while the light travels from 
M to 8” and back again, so oes the image is displaced in the 
eh of rotation of the mirr 
L 
s! s 
Let iy; be the ranma d of light; D, twice the distance M 8”; n, 
the number urns per second; 7, the distance M S and 6 the 
4nrn 
é 
deflection ; then V is found by the formula V= 
In a preliminary experiment the deflection amounted to five 
millimeters when the mirror revolved 128 times per second. 
IL GroLtoecy AND MINERALOGY. 
1. On the Limestones of the Falls of the Ohio, by Jamzs 
Hatt. 16 pp. 4to. Advance sheets of vol. ‘v, part 2, of the Paleon- 
tology of New York.—Professor Hall reviews the facts with ref- 
erence to the beds at the Falls of the Ohio v4 their fossils, gives 
the results of personal eS a s at the conclusion 
that en —_ include—beginnin - 
(1.) Niagara — Ty _ Upper Silurian, which are the “ Cate- 
nipora beds” of S$. 8 
(2.) Upper Helderbe deni to which bel ong the next follow- 
ing strata, (a) Coral sh (6) Turbo bed, (c) Nucleocrinus bed, 
d (d@) Spirifer bed, of Lyon. 
“@) — 30 feet of Hamilton beds, which are of impure mag- 
limestone, and comprise a the Hydraulic limestone, and 
the Encrinital limestone, of Lyon. 
(4.) The “ Black Slate,” which, after a special discuss sais of the 
ous intercalations a “ shales at its base, thins ree a 
—— the 
