On the dispersion of gaseous mercury, sulphur, phosphorous, and helium

Abstract In continuation of previous work on the refractive indices of certain elements in the gaseous state, we have measured the dispersion of the elements named above within the limits of the visible spectrum. Jamin’s refractometer was used, and the arrangement of the instrument was that described in our previous paper. But, as different wave-lengths had to be employed for the determination of the dispersion, the method of illumination was improved. Light from a Nernst filament was focussed on the slit of one of Messrs. Hilger and Co.’s fixed-deviation spectroscopes, and, in the focal plane of the resulting spectrum, a slit was placed capable of motion in that plane. The adjustment was calibrated by comparing the wave-length of the light passing through the slit with the reading of the drum of the spectroscope. By rotating the drum, light of any required wave-length could be obtained, and, by narrowing the slits, the spectrum was of sufficient purity to admit of two hundred bands being counted to one-fifth of their breadth.