A Powe rf'ul Spect roscope. In the young science of spectroscopy, as in others, an im pOl ·tant element of progress is the improvement of instru· ments for dealing with the phenomena presented, and many minds are engaged on this. A new spectroscope of remark able power has just been brought to the notice of the French Academy by M. Thollon. Its chief feature is the use of sul phide of carbon prisms, which are closed laterally, not by plates with parallel faces, but by prisms of thl) form of
... i's-i. e., having curved sides meeting at an angle \ which, however, is much smaller than Amici's prism). The refringent angles of these prisms are in an opposite direc tion to that of the sulphide prism. Two of these compound prisms are substituted by M. TholJon for the simple prisms in a spectroscope, which he formerly described to the Academy. Without going into further details, we may sim ply state that an enormous dispersion is obtained; with a magnifying power of 15 to 20 times, the spectrum has a length of 15 meters. The angular distance of the D lines of sodium is about 12', whereas that produced by M. Gassiot was only 3' 6". This instrument should throw considerable light on the struc1:'l1re of the spectrum, and M. Thollon has already noticed some interesting facts. The lines of sodium and magnesium present a dark nucleus passing into a nebu losity, which becomes gradually merged in the continuous spectrum. Many lines have been split up, and all that have been thus resolved have been found to belong to two differ ent substances. One of the hydrogen lines presents a nebu losity without a nucleus. M. Thollon remarks on the mag nificence of the spectrum of carbon from the electric arc, observed with the new instrument. The spectra of iron, copper, and magnesium in the same arc were aiso seen with admirable clearness and brilliancy. These new spectro scopes have been constructed for M. Thollon by the able optician, M, Laurent.