The refraction of magnesium oxide and its temperature variation

The temperature coefficient of refraction of magnesium oxide has been determined from room temperature to 400°C. at three wave-lengths, λ 4358, λ 5461 and λ 5893. The coefficients are found to be positive, as is to be expected from photo-elastic data. The values are discussed in the light of a dispersion formula for magnesium oxide newly proposed by the author, and are used to calculate the temperature shift of the effective characteristic frequency of the crystal in the ultra-violet.