44 0. N. Rood—Hye-piece Micrometer for the Spectroscope. 
an hour between the first and second, and half an hour between 
the third and last. 
At Cornwall, Canada, there were three shocks of considerable 
violence between 2 and 3 Pp. mM. At Moulinette there was a 
slight shock as 1 P. M., and a heavier one at 3" 20™ P. M. 
April 29, 1873. A sharp shock was felt at Doncaster, Eng. 
April 30, 1878. In the night a shock was felt at Cornwall 
and Cateau Landing, and also at Hamilton, Ontari 
rio. 
About 10.80 P. M., the same night, three shocks were felt 
Xx. 
May 8, 1873. About 3 P. M. a shock was felt throughout 
Gibsgp and Carroll Counties, Tennessee, and at Cairo, Til. 
For assistance in collecting the above, I am indebted to C. 
G. Rockwood of Newark, N. J., and to Edward L. Gaul and 
Jacob P. Thompson of the New York Times. 
Brunswick, Me., May 27. 
Art. X.—On a convenient Hye-piece Micrometer for the Spec- 
troscope ; by Prof. O. N. Roop. 
I HAVE recently contrived a very simple eye-piece microm- 
eter for the study of spectra produced by prisms and “ grat- 
ings,” which, while quite inexpensive, is capable of yielding 
results that are not easily surpassed except by the use of an 
eye-piece provided with a micrometer screw. A thin semi- 
circular plate of silver is made quite smooth, and rendered — 
bl 
y holding it over the flame of a lamp: it is afterward 
flowed with a drop of weak spirit-varnish to cause the lamp- 
black to adhere. Crossing the straight edge of this dead-black 
surface, lines 4™™, ete., are ruled with a dividing engine, and 
the necessary figures added with the help of a lens. T 
opaque semi-circular plate is then introduced into the interior 
of a negative, or preferably in front of a positive eye-piece, 80 
that it is in focus and does not occupy quite half of the field 
of view. Opposite it, and somewhat nearer the eye, an open- 
ing is made in the side of the eye-piece, whereby the lines are 
Pater! illuminated—as a general thing, merely by the dif: 
fused light of the room; but if this is quite dark, the small 
flame of a distant lamp easily accomplishes the same end. 
This arrangement, it will be seen, furnishes a set of bright lines 
on an almost perfectly black ground, with the least possible — 
outlay of expense or trouble in manipulation, and the degree of 
their brightness, it will be found, can readily be regulated 
merely by shading the opening more or less with the hand. 
The distanee of the lines apart should not be too small, as 18 
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