The initial object of this research was to find the dispersion of light in the monatomic gases argon and helium, but as it was necessary to know the absolute value of the refractive index with considerable accuracy, determinations of the refractive index have also been made. Their relative refractive powers (air = 1 ) have been found by Rayleigh, and Ramsay and Travers, using coloured interference fringes, but no determinations of the absolute refractive index for light of given wave-length have hitherto been made. The interferometer method due to Jamin was used. A horizontal beam of parallel white light was incident on the first Jamin plate. The two reflected beams traversed two brass tubes of equal length closed by equal thicknesses of worked plane glass. Each brass tube had a small side tube attached which led to apparatus for altering the density in the tube and measuring its pressure. After reflection at the second Jamin plate, the recombined beam was focussed on the slit of a spectrometer, the spectrum obtained with a plane diffraction grating was examined through the telescope of the spectrometer and was seen crossed by bands of maximum and minimum intensity. At the same time, by means of a small reflecting prism, a portion of the slit was illuminated by the light from a Plücker tube containing hydrogen, helium, and mercury vapour. Thus one observed simultaneously the interference bands and certain standard lines of known wave-length with any one of which the cross wire of the telescope could be brought into coincidence.