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 (X 5462-23) was found to be 1 -00006716. For the dispersion the cadmium red (X 6438) and blue (X 4800) lines were also used, and the results were found to be expressed by the formulawhere n is the frequency of the light = V/X. Only 200 bands could be counted, and the figures for the dispersive power were not believed to be trustw orthy to less than 5 per cent.In 1910f we remeasured the dispersion with greater accuracy, assuming the previous value of the refraction for X 5462-23. The number of bands counted was 440 and the dispersion was observed at seven points in the visible spectrum, between XX 6708-4800.The dispersion was best represented by _ I =

In a letter addressed to ‘Nature,’ in October, 1902, attention was drawn to the fact that the refractivities of the five inert gases of the atmosphere, He, Ne, A, Kr, and X, as determined by Ramsay and Travers, were, within narrow limits of accuracy, in the proportion of 1, 2, 8, 12 and 20; or, more simply, of 1/4,1/2, 2, 3, and 5.

I.On the Refraction and Dispersion of the , Halogen , S t e a m, Oxides of Nitrogen B y Cl iv e C u t h b e r t s o n ,Fellow o f University , London and M a u d e Cu t h b e r t so n .

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