Optical constants of liquids for short electric waves of length 4 to 27 mm were measured by use of special Hertzian oscillators and radiometer receivers developed by Nichols and Tear, for water, glycerin, and methyl and ethyl alcohol. For the extinction coefficient, glass and ebonite cells were used; for the coefficient of reflection, the liquid was held in a tilted rubber tray. The refraction indices were then computed by the Cauchy-Quincke formula. In a few cases the index was determined directly from measurements of the reflection from a thin layer of the liquid over a mercury surface as a function of the thickness, since, due to interference, maxima occur at intervals of half a wave-length, providing the absorption is not too great or the reflection too small. Water shows absorption maxima at wave-lengths of 20, 7 and perhaps 2 mm; its index drops from 8.7 for 40 mm to 5.3 for 4 mm. The experimental values agree with those computed by Rubens from the Debye theory for $\lambda >40\mathrm{}$ and $\lambda <0.2\mathrm{}$ mm; the discrepancies in the intermediate region are evidently due to the presence of two or three absorption bands, whereas only one was assumed.

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