Foucault, Jean

It often takes time for the implications of experimental data to be understood and to be acted upon. Fraunhofer s earlier observation that the solar D-lines coincided with the spectral lines of a sodium lamp eventually prompted further important experiments. In 1849, Jean Bernard Leon Foucault (1819-1868), a Parisian physicist, made an unexpected discovery. He passed sunlight through a vapor of sodium and he found that the solar D-lines were darker. His conclusion was that the sodium vapor presents us with a medium which emits the rays D on its own account, and which absorbs them when they come from another quarter. The consequences of Foucault s experiment, however, were expressed more cogently by Sir William Thomson (later Lord Kelvin). He drew the following explicit conclusion That the double line D, whether bright or dark, is due to the vapor of sodium. . . That Fraunhofer s double dark line D, of solar and stellar spectra, is due to the presence of vapor of sodium in atmospheres surrounding the Sun and those stars in whose spectra it has been observed. ... 

BMC4523 C. Ronco, G. Sorin, F. Nachon, R. Foucault, L. Jean, A. Romieu, and... 

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