178 SCIENCE PROGRESS 



glass. It was in the search for the best available combina- 

 tions that Fraunhofer made his careful determinations of the 

 refractive and dispersive powers of different kinds of glass. 

 In the process he rediscovered the dark lines of the solar 

 spectrum ; having satisfied himself that they depended on 

 the nature of sunlight and not on diffraction, he named the 

 more prominent lines by the letters of the alphabet A, B . . . . 

 H, I and utilised them to define the colours used in his 

 measures of dispersion. It is clear that he felt justified in 

 doing this only when he had assured himself that the lines 

 were not due to any kind of diffraction. In his experiments 

 with sunlight the light had to be passed through a narrow 

 opening and Fraunhofer was evidently suspicious of diffrac- 

 tional effects, until his observations of Venus, made without 

 any narrow opening, proved that the lines were indubitably 

 connected with the constitution of the light and that the light 

 of Venus was of the same nature as that of the Sun. He 

 went further and even examined the light of Sirius, finding 

 that its spectrum was different from that of the Sun and that 

 it was marked by the presence of three broad bands, one in the 

 green [F or H^] and two in the blue [H^ and H5 ?]. 



He moreover recognised the fact that the yellow line seen in 

 the spectrum of the flame of his lamp consisted in reality of two 

 fine bright lines which in intensity' and separation were similar 

 to the dark lines D in the solar spectrum. 



The memoir in which he communicated his results finished 

 with the words : " Da der hier mit physisch-optischen Versuchen 

 eingeschlagene Weg zu interessanten Rcsultaten fiihren zu 

 konnen scheint, so ware sehr zu Wunschen, dass ihm geiibte 

 Naturforscher Aufmerksamkeit schenken mochten." 



The next important advance was recorded in 1849 by Foucalt. 

 He had observed the spectrum of the electric arc and found 

 it " marked, as is known, in its whole extent by a multitude of 

 irregularly grouped luminous lines." In particular he noted 

 the double line at the boundary of the yellow, and rediscovered 

 the circumstance observed by Fraunhofer in 18 14, that the two 

 components coincided exactly with the D lines in the solar 

 spectrum. His ingenious method of throwing an image of the 

 Sun upon the arc itself allowed him to observe simultaneously 

 the electric and solar spectra superposed, and in the process 

 he made " some unexpected observations." Firstly it proved 



