IX. On the ultra-violet spark-spectra of some of the elements

Abstract

In the following paper an account is given of an investigation on the extreme ultra-violet spectra of a number of the elements. In the preliminary part of the work the spectrograph described by Prof. Mclennan* was employed, together with a Rowland grating of 100 cm. focal length, having 6273 lines per cm. and an area 5·4 cm. wide and 7·8 cm. high. The source employed was a water-cooled arc chamber in which the electrodes were tipped with the metals whose spectra were required. This apparatus proved to be unsuitable, for the reason that the spectrograph, as constituted, did not allow the source to be brought nearer the slit than about 1 foot. This resulted in very feeble illumination and consequently long exposures were required, which lasted in some cases for 10 hours. Such exposures were found to be necessary, not only because the source was too far removed, but also because the angular aperture through the slit included only a band across the centre of the grating about 2·5 cm. wide. Some of the modifications which were made in the spectrograph greatly improved its efficiency. A short description of these follows. Fig. 1 shows a horizontal plan and fig. 2 a vertical elevation of the spectrograph and spark chamber, as at present constituted. The body of the spectrograph A, B, was designed for a grating of 1-metre focal length and is about 130 cm. long over all. The enlarged portion A, which encloses the grating G and its mounting, is 22 cm. in diameter and 30 cm. long. The smaller portion B has a diameter of 15 cm. As shown in fig. 2 the opening above the grating is closed by a brass cap which fits into a tapered ground joint. The opening gives ready access to the grating for adjustment, and when the cap is placed in position, with the bearing surface slightly smeared with “Airtite” and the groove shown in the figure filled with the same, a perfect seal is obtained. The ead of the spectrograph nearest the grating is closed by a plate-glass window R, 2 cm. in thickness, set into a cast brass ring, the joint being made air-tight by a hard wax. This window enables one to see that the source, slit and grating are properly aligned.