After a few introductory remarks, the authors describe the apparatu which they employ, and their general method of observing the spectra the fixed stars and planets. The spectroscope contrived for these inqu ries was attached to the eye end of a refracting telescope of 10 feet foe length, with an 8-incli achromatic object-glass, the whole mounted equa torially and carried by a clock-movement. In the construction of th spectroscope, a plano-convex cylindrical lens, of 14 inches focal length, wa employed to convert the image of the star into a narrow line of light which was made to fall upon a very fine slit, behind which was placed an achromatic collimating lens. The dispersing portion of the arrangement consisted of two dense flint-glass prisms; and the spectrum was viewe through a small achromatic telescope with a magnifying power of between 5 and 6 diameters. Angular measures of the different parts of the spectrum were obtained by means of a micrometric screw, by which the position of the small telescope was regulated. A reflecting prism was placed over one half of the slit of the spectroscope, and by means of a mirror, suitably adjusted, the spectra of comparison were viewed simultaneously with the stellar spectra. This light was usually obtained from the induction spark taken between electrodes of different metals. The dispersive power of the apparatus was sufficient to enable the observer to see the line
Ni
of Kirchhoff between the two solar lines D ; and the three constituents of the magnesium group at
b
are divided still more evidently*. Minute details of the methods adopted for testing the exact coincidence of the corresponding metallic lines with those of the solar and lunar spectrum, are given, and the authors then proceed to give the results of their observations.
A Radiometric Technique for Monitoring the Desulfurization Process of Blister Copper