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The experiments of two Frenchmen, Hippolyte L. Fizeau and Leon Foucault, served finally to convince the last lingering sceptics that light is an undulation; and by implication brought heat into the same category, since James David Forbes, the Scotch physicist, had shown in 1837 that radiant heat conforms to the same laws of polarization and double refraction that govern light.
These observers did not double the canals satisfactorily, but all agreed upon the striking whiteness and brightness of the planet. M. Fizeau argued that the Schiaparelli canals were really glacial phenomena, being ridges, crevasses, rectilinear fissures, etc., of continental masses of ice. In 1889, Prof.
The history of the method of measuring star diameters is a very curious one, showing how the most promising opportunities for scientific progress may lie unused for decades. The fundamental principle of the device was first suggested by the great French physicist Fizeau in 1868.
Thus Fizeau, the first to have this idea, says: "A ray of light, with its series of undulations of extreme tenuity but perfect regularity, may be considered as a micrometer of the greatest perfection, and particularly suitable for determining length."
By an experiment proposed by Arago, and executed with consummate skill by Foucault and Fizeau, this question was brought to a crucial test, and decided in favour of the theory of undulation. In the present instance also the two theories are at variance.
Still later, Leverrier made it 91,759,000. Airy and Stone, by another method, made it 91,400,000; Stone alone, by a revision of the old observations, 91,730,000; and finally, Foucault and Fizeau, from physical experiments, determining the velocity of light, and therefore in their nature altogether differing from transit observations, 91,400,000.
With his large Nicol prisms he in the first place repeated and explained the experiments of Foucault and Fizeau, and subsequently enriched the subject by very beautiful additions of his own. I here append a portion of the abstract of his discourse:
Fizeau led the way, and he was succeeded, after a few months, by Léon Foucault, who, in 1862, had so far perfected Wheatstone's method of revolving mirrors, as to be able to announce with authority that light travelled slower, and that the sun was in consequence nearer than had been supposed. Thus a third line of separate research was found to converge to the same point with the two others.
Thus the speed of light which has been measured by observers such as Fizeau and Foucault reveals itself as a function of the gravitational constant of the earth, and hence has validity for this sphere only.1 The same is true for Roemer's and Bradley's observations, none of which, after what we have stated earlier, contradicts this result.
De la Rue's new observatory, built in that year at Cranford, was expressly dedicated to celestial photography; and there he applied to the heavenly bodies the stereoscopic method of obtaining relief, and turned his attention to the delicate business of photographing the sun. A solar daguerreotype was taken at Paris, April 2, 1845, by Foucault and Fizeau, acting on a suggestion from Arago.
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