Companion to Sirius.—The Spectroscope. 407 
Art, XLII.—Scientific Correspondence. 
I. Letter on Companion to Sirius, Stellar Spectra and the Spectroscope, 
rom Lewis M. RurHerrurD, dated 175 Second Avenue, New York, 
March 31, 1863. 
Gentleme 
1, Compan anion to Sirius.—The poppet and Sane of the c 
954. Last year, the position resulting from a mean of Pass zi 
measures, on six nights, mean epoch March 28, was 84° 58! 46”, while 
twenty-eight measures of distance gave 10” 09, From a comparison 
of these results, it appears that, while the change in distance, 0-55, is 
so small that its existence cannot be asserted with oe ae a marked 
change of position has taken place, amounting to 3° 37’, reac 
so decided that the motion m 2 taken as flly wished: 
this rate of motion, assuming circular and in a plane perpen- 
in about 100 years, or, [ baborat twice as long as the period ascribed to 
ed to 
orbit attributed to the opaque body supposed to dei: the great star, 
but I have been unable to lay my hand upon the of Bessel and 
Peters upon this subject, in time for this letter. 7 still wonder that 
Clark’s great little star has so long escaped detection; it is a much less 
t 
2 tdlar ectra ctnes writin ou in December (p. 71, this 
volume), I have mounted my siceoomien spectroscope in a more firm 
d convenient manner; I have added m, by means of which the 
spectrum from a spirit lamp is constantly present in the field of view, 
during the observation of a star: I find a most useful check, and 
Mainty, found that each line in the spectrum of the star has its counter- 
in the solar spectrum. 
_ 8. The Spectroscope. _~ I have employed the bad weather, this winter, 
in the construction of a large nt Cage te age eg 20 inches focus 
and 1°6 aperture; the sae of which I have so far used but six, are 
hollow cases of brass cast in one piece, with Ruse Sores carefully ground, 
Upon which are camaiad plates of glass, originally made for shades for 
artificial horizons, and consequently nearly plain and parallel; I say 
nearly, for I have yet to find one aan tie of plain and parallel gine ; 
these prisms under certain conditions perform beautifully ; the obstacles 
