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POPULAR SCIENCE REVIEW. 
two poles the same degree of activity in the case of unipolar excitement of 
the motor nervous bundles. The contractions produced by positive and 
negative excitement with this typical intensity of the current are equal, 
both in magnitude and duration. Below this intensity equal currents pro- 
duce unequal effects with the two poles, the activity of the negative pole 
being more considerable. When tetanisation is produced by feeble currents, 
it is never with the positive pole upon the nerve. Above the typical value 
of the intensity of the current the inequality appears in the opposite 
direction. 
METALLURGY. 
Samarskite in America. — At a recent meeting of the Academy of Philadel- 
phia, Mr. Joseph Willcox called attention to a mineral specimen (samarskite) 
presented by him, which was found at a locality discovered by him recently 
among the mountains in Mitchell Co., North Carolina. Excepting in North 
Carolina, this rare mineral is only found in the Ural Mountains in Asia. 
According to Dana’s u Mineralogy,” the largest specimens found at the latter 
locality are only as large as hazel-nuts ; but Mr. Willcox said he obtained 
a specimen in North Carolina that weighs more than twenty pounds. It 
was associated with decomposed feldspar. 
The Association of the Native Platinum of the Urals. — A writer in 
the “ Geological Magazine,” whom we rake to be Professor Morris, says 
that M. Daubree, in an interesting paper before the Academy of Sciences, 
has shown that native platinum, although obtained abundantly in the 
alluvial deposits of certain regions of the Ural, has been found in a Peri- 
dote (Olivine) rock, which is more or less altered into serpentine, and accom- 
panied with diallage (a ferruginous sahlite, according to M. Des Cloiseaux), 
and also with chromite, which occurs abundantly, not only in separate grains, 
but also encrusting the grains of platinum. The platinum, which is here 
associated with chromate of iron, appears to be distinguished from the pla- 
tinum of other deposits by the large proportion of metallic iron with which 
it is alloyed. It appears that platinum very rich in iron, and endowed with 
magnetic polarity, has not been found — at least, at present — save in company 
with chromate of iron. 
MICROSCOPY. 
The New Platyscopic Pocket Lens promises to have a very great success, 
for it certainly completely distances the Stauhope or Codrington in largeness 
and flatness of field, if not in magnifying power. It is certainly the best 
pocket lens that we have ever seen, and we have very carefully examined 
and tested it before expressing this opinion. It is a triple achromatic com- 
bination, in which the chromatic and spherical aberrations are corrected by 
the central lens of dense flint. This lens is nearly three times as thick as 
the crown-glass lenses. The interior curves are almost hemispheres. The final 
correction of spherical aberration is made by altering the thickness of the 
