The theory of optics / by Paul Drude ; translated from the German by C. Riborg Mann and Robert A. Millikan.

Date:
1902
Catalogue details

Licence: In copyright

Credit: The theory of optics / by Paul Drude ; translated from the German by C. Riborg Mann and Robert A. Millikan. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    148/612 (page 120)
    In spite of this extraordinary velocity with which light travels, a velocity 900,000 times greater than that of sound in air, the time required for light to travel astronomical distances is sometimes considerable. This appears, for instance, from the observations of Romer, which show that it requires 8 minutes for light to travel from the sun to the earth. Since many years are required for the light of the nearest fixed stars to reach the earth (from a Centauri 3J years, from Sirius 17 years), these great interstellar distances are usually reckoned in light-years. 5. Dependence of the Velocity of Light upon the Medium and the Color.—The velocity of light is independent of the intensity of the source. This has been proved by very delicate interference experiments made by Lippich and Ebert. On the other hand the velocity does depend upon the medium in which the light is propagated. Foucault compared by his method the velocities in air and in water by placing two mirrors sx and s2 in front of the rotating mirror m and inserting between m and s2 a tube of water 2 m. long. It was found that when the mirror m was rotated, the image reflected from the mirror s2 experi- enced a greater displacement than that reflected from sx, a proof that light travels slozver in water than in air. Quantitative measurements of the velocity of light in water and in carbon bisulphide have been made by Michelson. For the ratio of the velocities in air and in water he obtained 1.33; in air and in carbon bisulphide, white light being used, 1.77. The first number agrees exactly, the last approximately, with the observed indices of refraction. It is assumed (and in fact the wave theory demands it) that this result holds for all bodies. Hence the velocity of light in air must be somewhat smaller than in vacuum, since the index of air n — 1.00029. The number given above for the velocity of light which was obtained as a mean from the methods using terrestrial distances was reduced to vacuum by means of this factor. Since the index of all transparent media is smaller for the red rays than for the blue, it is to be expected that the veloci-