SIR WILLIAM HUGGINS, K.C.B., O.M. 179 



to him the extreme transparency of the arc ; and secondly it 

 showed him 



" that this arc placed in the path of a beam of solar light 

 absorbs the rays D, so that the line D of the solar light is 

 considerably strengthened when the two spectra are exactly 

 superposed. When, on the contrary, they jut out one beyond 

 the other, the line D appears darker than usual in the solar 

 light and stands out bright in the electric spectrum, which 

 allows one easil}^ to judge of their perfect coincidence. Thus 

 the arc presents us with a medium which emits the rays D on 

 its own account and which at the same time absorbs them 

 when they come from another quarter. To make the experi- 

 ment in a manner still more decisive, I projected on the arc 

 the reflected image of one of the charcoal points, which like 

 all solid bodies in ignition gives no lines (only a continuous 

 spectrum), and under these circumstances the line D appeared 

 to me as in the solar spectrum." 



Thus Foucault had hit upon the secret of the solar spectrum 

 in 1849. He had however to express it in terms of the ambiguous 

 word " medium." For, in spite of Sir John Herschel's studies of 

 the spectra of coloured flames in 1822, in spite of Fox Talbot's 

 study in 1826 of the bright lines in the spectrum of red fire, 

 and in spite of W. A. Miller's investigations in 1845 of the 

 spectra of the alkaline earths, it was not then clear that the 

 yellow D lines were due to sodium. It was not till 1856 that 

 Swan's experiments showed that the persistency with which that 

 pair of yellow lines appeared in all spectra was in great measure 

 due to the extraordinary delicacy of the spectroscopic test. An 

 excessively minute trace of sodium was enough, as he then 

 showed, to produce the "flame reaction," as it was called. 



Stokes had been one of the few who had realised the 

 importance of Foucault's work. When Kirchhoff's work was 

 published in 1859, it was he who had called attention to this 

 work by publishing a translation of Foucault's paper in the 

 Philosophical Magazine in i860. Stokes added to the translation 

 a few paragraphs showing how he himself attached a dynamical 

 significance to Foucault's " medium," regarding it as consisting 

 essentially of numberless systems, vibrating or capable of 

 vibrating with definite periods, which could make themselves 

 felt either by the emission or by the absorption of ethereal 

 waves. 



Thus, ten years after Foucault's work, the secret of the solar 



