The carbon arc spectrum in the extreme ultra-violet

Abstract

Considerable attention has been given in the last few years to the carbon spectrum in the extreme ultra-violet, that from an arc being investigated with a fluorite spectrograph by Wolff and McLennan, Ainslie and Fuller, and with a grating spectrograph by McLennan and Lang and McLennan and Petrie, while Millikan has measured the hot-spark spectrum to 360 A. U. On the whole, the agreement of these various workers as to the wavelengths of the lines is not very good, although McLennan and Petrie are in substantial agreement with Millikan concerning the most intense lines. It was thought worth while therefore to take an opportunity which offered to make another independent measurement of the wave-lengths of a number of lines in the arc spectrum, with the result that some have been photographed in the Lyman region which have been mentioned by no previous worker with the arc, and by the study of some of the lines in higher orders of spectrum, they have been shown in many cases to consist of close groups of lines. 2. Description of Apparatus. The vacuum grating spectrograph used is shown in outline in fig. 1. It consisted of a metal tube with chamber at the side for the introduction of a drying agent. The grating was mounted in a carriage provided with means for giving the grating a limited rotation about each of three mutually perpendicular axes. The carriage could be moved along slides, fixed near to one end of the tube, by a screw, so as to allow of accurately focussing the spectrum upon the photographic plate. This end of the spectrograph was closed by a metal plate. To the other end of the tube was soldered a casting carrying photographic plate and two slits. The position of the plate was fixed, the plate-holder being cylindrical and having the surface against which the plate was held worked to the same radius of curvature as that of the grating. The slits were placed immediately beneath the dark slide and recessed so that each lay on the same focal surface as the grating and photographic plate. Their distance apart was such that a range of spectrum of about 800 A. U. was obtained from either slit, these ranges overlapping by rather more than 80 A. U. for the same setting of the grating, and the focus of the spectrum being equally sharp whichever slit was employed. A shutter, operated by a permanent magnet mounted externally in a rotating collar, enabled two exposures to be made on the photographic plate which measured 6 cm. x 1 cm.