The absorption spectrum of cæsium

Measurements of the wave-lengths of the lines in the absorption spectrum of cæsium were first made by Prof. P. V. Bevan in 1912. He produced the vapour of cæsium in a steel tube by heating the anhydrous chloride with metallic lithium, and analysed the light of a carbon are passed through the vapour with a Hilger quartz spectrograph giving the spectum from λ 6000 - λ 2200 on a 10 inch plate. The wave length of 31 lines of the principal series were given to two places of decimals. His estimated error, howevere, in most of the lines amounted to 0·2 Å. U. These appear to be the only measurements extant of the principal series in the absorption spectrum of this element. In the present research an attempt has been made to obtain more accurate values by photographing the spectrum, using a Rowland grating of 10 feet radius of curvature and 20,000 lines to the inch. The ruled surface was 8 × 5 cm., but of the 8 cm. it was found that three had to be covered up to obtain the best definition. The spectrum used was the first order, and in the region of the series photographed the dispersion was about 43 Å. U. per centimetre, and the theoretical resolving power 0·08 Å. U.