292 [Dec. 1, 



In the course of these researches Professor Kirchhoff made the 

 remarkable discovery that flames which of themselves copiously emit 

 rays of definite refrangibility, and consequently exhibit bright lines 

 in their spectra, act at the same time as absorbing media of such a 

 character as to stop rays of those precise degrees of refrangibility, 

 when light containing rays of all kinds is transmitted through them. 

 Accordingly when a bright source of light, which of itself gives a 

 continuous spectrum, is viewed through such a flame, and the mixed 

 light, consisting partly of the light emitted by the flame, and partly 

 of the light transmitted through it, is analysed by a prism, instead of 

 a bright line on a dark 'ground, there is seen a dark line on a bright 

 ground, occupying exactly the same place. In order, however, that 

 this inversion should be observed, it was found to be essential that 

 the temperature of the bright source should exceed that of the flame ; 

 otherwise, in comparing the illumination of the place of the line in 

 the spectrum with that of its neighbourhood, the loss by absorption 

 would not overbalance the gain by emission. 



These results Professor Kirchhoff showed to be a necessary con- 

 sequence of Prevost's theory of exchanges, taken in an extended 

 sense ; so that, from the appearance of a bright line in the spectrum 

 of an incandescent vapour (for it is only in the state of vapour that 

 incandescent matter emits rays of definite refrangibility), it might be 

 inferred that the chemical substance in the vapour to which the 

 bright line was known to be due, would also in the state of vapour, 

 at the same temperature, act as an absorbing medium capable of 

 exhibiting a corresponding dark line in the spectrum of light trans- 

 mitted through it, an effect which the vapour would still continue 

 to exhibit at a lower temperature, unless the mode of absorption were 

 changed by so reducing the temperature. 



This doctrine finds a striking application in the explanation which 

 it affords of the existence of dark lines in the solar spectrum. The 

 exact coincidence of the double line D of Fraunhofer with a corre- 

 sponding bright line seen in many flames was pointed out by Fraun- 

 hofer himself; and a similar coincidence was observed by Sir David 

 Brewster between a system of bright lines shown by deflagrating 

 nitre, and a corresponding group of dark lines in the solar spectrum. 

 The theory of Professor Kirchhoff leads us to expect such coin- 

 cidences beforehand, and from the presence or absence in the solar 



