Abstract: Humidity is measured by alternatively producing two laser beams of different wavelengths at each of which the second differential derivative of the light absorption spectrum of water molecules is zero, dividing each laser beam, passing one divided beam along an optical path of known length through air, passing the other divided beam along an optical path of the same length through vacuum, causing the divided beams to interfere so as to obtain an apparent difference in length between the two optical paths, this difference being due to the difference in the indices of refraction between air and vacuum, calculating the index of refraction of the air at each wavelength from this difference, and calculating the density of water molecules in the air, i.e. the humidity of the air, from the so-obtained indices of refraction.