THE VELOCITY OF LIGHT.

Daily Telegraph (Napier), 3 August 1881, Page 3

THE VELOCITY OF LIGHT.

At tho Physical Society on Saturday last Professor George Forbes gave as interesting account of the recent experiments made by him, and Dr Young, of Glasgow, to determine the velocity of lighr-. The scene of these experiments was Wemyss Bay, on the Clyde, and the light was flashed across from there to Innellan, three miles distance, and reflected back again. The method employed was a modification of the well known plan of Fizeau, in which a ray of light is sent through the space between two teeth of a revolving wheel and reflected back again ; while the wheel is rotated so fast that the reflected ray is eclipsed by the succeeding tooth. The velocity of tho ray is then found from tho distance ia baa traversed and the

angular -velocity 6? thb Wheel. Instead of totally eclipsing the ikf; Messrs Forbes and SToung, for greater accuracy, employed two I' mirrors, ono a quarter of a mile behind the other, and the speeds of the toothed wheel, 1 which rendered each reflected ray apparently of equal brightness to the eye, were carefully observed. , This comparison of lights was found more reliable than the observation of an entiro occupation of a single riff. _ A delicate electrical chronograph marking time correctly to the ten-thousandth part of a second was employed ; the light was furnished by an electric lamp. The general result of the experiments is that light travels at the rate of 187,200 miles per second. Cornu's result is that light traverses at the rate' of 186,700 miles per second, and Michaelson the American's determination is 186,500 miles. Cornu's result was, however, obtained by the light of a petroleum lamp and Michaelson's by sunlight, whereas that of Professor Forbes was got with an electric lamp. It is higher than either of the other two, and Prosessor Forbes was led to believe this due to the fact that there are more blue rays in the electric light. A number of experiments have led him and his colleague to the_ conclusion that blue light travels at a slightly faster rate than red light. "With lights coloured by solutions, glasses, and the refraction of the prism, they have found the velocity of blue light to be over 1 per cent (I*7) greater than that of red light. This result if verified, is a very important one, and as Mr Spottiswoode, President of the Royal Society, remarked, it will tend to modify our ideas of the constitution of the luminiferous ether. It follows from it, too, that variable stars will show a blue tinge as they wax in brilliancy, and a red tinge as they wane ; but it hardly ensues, as Lord Rayleigh seemed to think, that tho satelites of Jupiter would appear blue when they emerge behind their primary after an eclipse, for this emergence is comparatively slow. It should be added as Professor Forster pointed out, that any effect of dispersion of the light by air in those experiments would be in tho direction of retarding the blue rays more than the red. —Engineering,