Ixxxiv 



disk being made to revolve rapidly in its own plane, on bringing a horse- 

 sboe magnet into such a position that the disk revolved with its rim be- 

 tween the poles of the magnet, the moving force required to maintain the 

 velocity of rotation increased, and the temperature of the disk was raised. 



On the 16th of February 1857 he described a reflecting telescope, 

 having a speculum formed of glass coated with chemically reduced silver 

 and afterwards polished, of 10 centims. aperture and 50 centims. focal 

 length, without being aware that a telescope on the same principle and 

 nearly of the same dimensions had been described by Steinheil in the 

 Allgemeine Zeitung of the 24th of March 1856. In the following year 

 Foucault succeeded in giving the speculum the form of a spheroid or of a 

 paraboloid of revolution, and described a new process for finding out the 

 configuration of optical surfaces. A reflector of this description, having 

 an aperture of 40 centims. and 2*5 metres focal length, was mounted in the 

 Imperial Observatory of Paris in June 1859. Another of these reflectors, 

 having an aperture of 78 centims. and a focal length of 4'5 metres, was 

 constructed for the Observatory in 1862. The polarizer known as his was 

 invented in 1857. 



The project of determining the absolute velocity of light in air with the 

 aid of Wheatstone's revolving mirror, conceived in 1850, was carried out 

 in 1862. The value Foucault obtained for it was 298,000 kilometres in a 

 second of time, instead of 308,000 kilometres, the previously received value. 

 Combining the newly found velocity with the constant of aberration, 20"445, 

 the sun's equatorial parallax is found to be 8"'86, the value deduced by 

 Mr. Stone in his recent discussion of the transit of Venus in 1769 being 

 8"'91, and the value adopted in the 'Nautical Almanac' for 1870 being 

 8''- 95. In this year Foucault was elected a Member of the Bureau des 

 Longitudes. 



In the years 1863, 1864, 1865 he appears to have been occupied with 

 the task of investigating the conditions of isochronism of Watt's governor, 

 and modifying its construction so as to render the time of revolution inva- 

 riable. These improved governors are applied to the transit-recorders con- 

 structed for the use of the Indian Survey. In January 1865 he was elected 

 a Member of the Mechanical Section of the Institute. In 1866 he invented 

 a new and improved regulator for the electric light, and a telescope for 

 viewing the sun, in which the light is rendered endurable to the eye by 

 coating the outer surface of the object-glass with a film of chemically re- 

 duced silver so thin as to be transparent. This process was applied with 

 complete success to a refractor having an aperture of 25 centims. 



In July 1867 he was attacked by paralysis, and died on the 11th of 

 February, 1868. The date of his election as Foreign Member of the 

 Royal Society is June 9, 1864. 



