In the Proc. Roy. Soc. Edin. (Houstoun, 1941) I described a new method of measuring the velocity of light. Owing to the difficulties of the times progress has been slow, but I have now succeeded in measuring the velocity of light in water and communicate the result here.
Previous Measurements.—Every elementary textbook on Light explains how Newton's, corpuscular theory required the velocity of light in water to be μ times the velocity in air, whereas Huygens's wave theory required it to be I/μ times the velocity in air, and how Foucault's determination decided between the two theories. As a matter of fact it did, but Foucault's determination was not an accurate one and there have been only four previous determinations of the velocity of light in water, none of which can lay claim to accuracy. These are the determinations of Foucault (1850), Fizeau and Bréguet (1850), A. A. Michelson (1891), and Gutton (1911).