Ixxi 



ately above the other; the location of the colors in both exactly coinciding. 

 He then brought the vapor of burning sodium in the path of the electric 

 beam, and immediately a bright yellow line was seen in the electric spec- 

 trum exactly coinciding in position with the darkD line of Frauenhofer in 

 the solar spectrum. (This line is located between the orange and the 

 yellow^, and it has since been found to be composed of two distinct bands.) 



When magnesium, iron, calcium, and several other metals, were burned 

 separately, each one marked the electric spectrum with differently located 

 threads of bright light, exactly corresponding in position with dark threads 

 in the solar spectrum. Some of these threads were located in one color 

 of the spectrum and some in others. Some of the metals produced several 

 threads of light in two or three colors, as for instance iron, which has al- 

 ways several threads in the green and two in the violet. When the beam 

 was passed through the vapor of several. different metals at the same mo- 

 ment, no confusion occurred, but the lines of each metal were distinctly 

 visible and each in its appropriate place in the spectrum. This discovery 

 established the fact that there was a direct connection between the causes 

 which produced these threads in the solar spectrum and those which 

 caused them to appear at the will of the experimenter in the spectrum Of 

 the electric light. But in the one they appeared as dark lines, and in the 

 Other as bright ones. Here was a discrepancy that had to be explained 

 before it could be asserted positively that the dark D line, for instance, in 

 the solar spectrum was produced by incandescent sodium in the sun, just 

 as the same bright line was caused in the electric spectrum by the vapor 

 of burning sodium. 



I explained that the waves of light are absorbed with great energy by 

 matter whose periods of molecular vibration concur with the periods of 

 the ethereal waves. A body therefore absorbs with special facility such 

 rays as it can itself emit. If we bear in mind that the energy of the vibra- 

 tions of matter generally affect only their amplitude, and not their rapi- 

 dity, we will see that incandescent sodium will emit rays whose periods of 

 vibration are the same as those emitted by the gaseous flames of sodium, 

 and that if the rays of incandescent sodium be transmitted through sodium 

 vapor, they must be absorbed by the vapor. 



This remarkable fact was demonstrated by Kirchhoff by interposing the 

 vapor of burning sodium in the spectrum of incandescent sodium, by 

 which the bright D lines became at once black. 



The inference is therefore unavoidable that the nucleus of the sun is 

 composed of certain incandescent metals, some of the rays from which are 

 absorbed by the gaseous products of the same metals surrounding the sun. 



If they could come to us without being intercepted, the solar spectrum 

 would appear uninterrupted by these dark lines. These absent rays are 

 absorbed by the vapors of the different metals which emit the rays, and we 

 have in their stead only the fainter rays emitted by these different vapors 

 themselves. These really produce bright lines also, but they appear dark 

 in the spectrum because of the presence of the more intensely brilliant 

 ones which reach us without interception from the nucleus of the sun. 



