SECT. iv. REVERSION OF THE COLOURED LINES. 149 



others ; thus the coincidence of the spectra of volatilized 

 substances with the solar line forms a regular system. 



Professor J. P. Cooke, junior, has recently constructed 

 a spectroscope which shows that the lines of the solar 

 spectrum are as innumerable as the stars of heaven, 

 that at least ten times as many are distinctly seen as 

 are given by Kirchhoff in his chart, besides an infinitude 

 of nebulous bands just on the point of being resolved. 

 Yet even with this greatly increased power, the coinci- 

 dences between the bright lines of the metallic spectra 

 and the dark lines of the solar spectrum remain perfect. 

 M. Kirchhoff had seen a fine yellow line between the 

 double lines D of the sodium spectrum. M. Merz of 

 Munich found four additional lines, but Professor Cooke 

 has discovered that there are in all seven intermediate 

 lines and a nebulous band. Although the two members 

 of the sodium line D could be spread so far apart that 

 the -3 oV o P ar ^ f * ne intermediate space could be readily 

 distinguished, yet the coincidence with the two dark 

 Fraunhofer lines was absolute. The spectroscope 

 ' shows that many of the bands of the metallic spectra 

 are broad coloured spaces crossed themselves by bright 

 lines. This is the case with the orange band of the 

 strontium spectrum, and with the whole of the calcium 

 and barium spectra to a remarkable extent.' 2 



As early as the year 1849, M. Foucault discovered that i 

 the sun's light when shining through the electric lighfrl 

 gives black bands on that part of the spectrum where \ 

 the electric light alone would have produced bright bands, 1 

 so that the black and bright bands could be produced \ 

 alternately by admitting or excluding the solar light ; 

 whence he concluded that the electric arc emits the same 

 lines which it absorbs when they come from another lu- 

 minous source. M. Angstrom also observed that the 



* The Chemical News and Journal of Physical Science for July 4, 1863. 



