484 HISTORY OF SCIENCE. 



ride of tungsten, which have the same appearance, give, as W. H. 

 Miller discovered in 1845, no absorption lines whatever. The ab- 

 sorption lines are seen in the vapour of some chemical elements and 

 not in that of others. Compound bodies in the state of vapour may 

 exhibit absorption lines, while their constituents do not, or vice versa. 

 Among the other observations made by Fraunhofer in 1814 was 

 one the significance of which was not understood until many years 

 afterwards. He found that the conspicuous dark line of the solar 

 spectrum, designated by the letter D, was in reality formed by two lines 

 very close to each other ; and he noticed and this is the fact which 

 was afterwards to become the starting-point of a new path of inquiry- 

 that these two dark D lines were identical in their position in the 

 spectrum with two bright lines which were seen in ordinary flames. 

 These lines having since been proved to be due to the presence of 

 sodium^iQ now often called the sodium lines. Again, in 1849, Foucault, 

 while viewing these bright sodium lines as displayed in the rays from 

 the electric light, caused the concentrated rays of the sun to traverse 

 the electric arc, and he observed that with this compound light the 

 dark sodium line of the solar spectrum appeared more intense than 

 usual. He drew no general inference from this fact, and although 

 Professors Stokes and W. Thompson had in the meantime pointed 

 out that these and other facts pointed to certain general principles, it 

 was reserved for two eminent German men of science to investigate 

 the relations between the bright and the dark lines, and to propound 

 a general theory of these phenomena. Professor Kirchhoff, of Heidel- 

 berg, desired to put to the most direct test the coincidence of the 

 bright sodium lines with the dark Fraunhofer D lines of the solar 

 spectrum. When he had obtained a moderately bright spectrum 

 without admitting the sunlight directly into his spectroscope, he 

 brought a flame coloured by a sodium compound in front of the slit 

 of the spectroscope. He then saw the bright sodium lines due to the 

 flame appear in the very same places that had been occupied by the 

 D lines. Next he allowed the direct rays of the sun to enter the slit 

 after passing through the sodium-flame, and he was surprised at the 

 greit intensity of the black D line which then appeared. Continuing 

 tne inquiry, he now began to vary the condition of the experiment. The 

 sunlight was replaced by the oxy-hydrogen limelight, which, like every 

 incandescent solid, furnishes a spectrum perfectly continuous, that is, 

 without either bright or dark lines. When the light from the incan- 

 descent lime was received into the spectroscope after passing through 

 a flame coloured by common salt, two dark lines were seen in the 

 positions of the sodium lines. In this last experiment it is plain that 

 the rays which in the absence of the limelight produced the bright 

 sodium lines, also entered the slit of the spectroscope along with the 

 much more intensely luminous rays from the limelight, and the ap- 

 pearance of darkness arose from the more intense illumination of the 



