So far as we are aware only four determinations of the refractivity of argon have been published. These are as follows:— The first two determinations only deal with the refraction of white light, while the value of Ahrberg’s results is limited by the fact that he was only able to count 12 bands in the red and 50 bands at the other points selected; so that his whole dispersion effect must have been about one band in the violet, a number which is insufficient to ensure great accuracy. Another determination may not, therefore, be considered superfluous. The specimen of argon used by us was kindly lent by Sir William Ramsay, by whom it was purified. The spectrum showed a trace of hydrogen, but none of other gases. The mean of seven experiments gave a refractive index of 1.00028230 for the wave-length 5461. These determinations varied between 1.00028258 and 1.00028183. The dispersion was measured for the seven wave-lengths 6438.5 (Cd), 5790.5 (Hg), 5769.5 (Hg), 5460.7 (Hg), 5209.1 (Ag), 5085.8 (Cd), 4799.9 (Cd).