The refraction and dispersion of neon and helium

Some apology is needed for a third paper, by the same authors, on the refraction and dispersion of neon. In 1909 we published determinations made with 300 c. c. of gas purified by H. E. Watson for Sir William Ramsay. The index for the green mercury line (λ 5462·23) was found to be 1·00006716. For the dispersion the cadmium red (λ 6438) and blue (λ 4800) lines were also used, and the results were found to be expressed by the formula μ – 1 = 2·5665 ⨯ 1027 / 38517 ⨯ 10 27 – n2, where n is the frequency of the light = V/λ. Only 200 bands could be counted, and the figures for the dispersive power were not believed to be trustworthy to less than 5 per cent.