~~ 
EL. W. Hiigard—Geology of Lower Louisiana. 77 
of the pupil is most intimately associated with the focal ad- 
‘4G justment. 
rays must come to the eye as if they diverged from the 
same distance; for the eyes are adjusted for that distance. To 
fulfil this condition lenses are always used ; but it is obvious 
) that a given pair of lenses are suitable for one distance only. 
)  Forall other distances, or degrees of optic convergence, there 
and this is both difficult and fatiguing to most persons. 
_T have found that observations upon the images of the ruled 
Dah ES SE ee ee eT 
ES ae ee 
tations of the eye, and the position of the Horopter. I hope in 
| ‘WY next communication to take up this most difficult subject. 
(To be continued.) 
Arr. VII.—On the Geology of Lower Louisiana and the Rock- 
salt Deposit of Petite Anse; by Lucene W. Hinearp, Pu.D., 
of Oxford, Miss. (Abstract.) 
Tue discovery in 1862, of a deposit of rock-salt on the coast of 
Louisiana, was a fact so unexpected to geologists, that at any 
er time a detailed investigation cf its geological relations 
would quickly have followe The press- 
ing necessities of the blockaded section soon caused its exploita- 
tion on the large scale, though in a very irregular manner; fora 
Considerable period, these mines supplied the whole of the 
In November, 1865, Prof. Richard Owen made a 
| Utief examination of the locality, the results of which — 
_ lished in the Transactions of the St. Louis Academy. A year 
must be some degree of dissociation of the two adjustments, . 
m are a most delicate means of determining both the ro- | 
