The infra-red secondary spectrum of hydrogen

Abstract

The first observations of the "many lined" spectrum of hydrogen in the infra-red made by Croze, who measured the wave-lengths of 72 lines between 6838 Å. U. to the nearest on the Rowland system. Porlezza measured the wave-lengths of 43 lines between H a , though his plates were treated to record up to 8000 Å. U. Croze corrected some of his earlier lines and added some 27 more, but did not extend the further limit beyond 8027 Å. U., and his later results were still only given to the nearest integer. As Merton and Barratt had investigated the secondary spectrum only for wave-lengths less than H a , it was thought desirable to re-investigate the infra-red region, to obtain a more accurate record of the wave-lengths of the lines, and to push the limit, if possible, to longer wave-lengths. A plain diffraction grating spectroscope fitted with quartz lenses was used. The grating had 14,500 lines to the inch and gave a dispersion of 25 Å. per mm. An H-shaped vacumm-tube, fitted with aluminium disc electrodes at the heads of the upright stems, contained hydrogen, and to one stem of the tube a large glass bulb was sealed, so that a considerable volume of gas was available at about 2 mm. pressur. The capillary tube connecting the stems was 2 mm. internal diameter, and light emerging from the end of this tube parallel to the tube was directed on to the slit of the spectroscope with a quartz lens. In this "end-on" position the intensity of the secondary spectrum was at a maximum.