14 Prof. Kirchhoff on the Relation between the Radiating and 



constant the function I can have no strongly marked maxima 

 and minima for waves of different lengths. Hence it follows that 

 if the spectrum of a red-hot body presents discontinuities or 

 strongly marked maxima or minima, the power of absorption of 

 that body, regarded as a function of the length of the waves, must 

 present similar discontinuities or strongly marked maxima and 

 minima. Spectra with strongly marked maxima may be ob- 

 tained by placing various salts in the flame of a Bunsen's lamp. 

 Chloride of lithium affords interesting results in this respect. If 

 a bead of this salt be melted in a platinum ring and placed in the 

 mantle of the gas-flame, the spectrum of the flame (when 

 unaffected by the presence of other salts and not too brilliant) is 

 a single bright red line of light, formed of waves whose length is 

 about the arithmetic mean of the lengths of the waves corre- 

 sponding to the lines B and C of Fraunhofer. For waves of this 

 length the radiating power of the flame is very considerable, 

 while for waves of lengths corresponding to the other visible 

 colours it is imperceptible. Accordingly, the power of absorption 

 of the lithium-flame must be great for waves of this length, but 

 very small for those constituting the other visible rays. If, there- 

 fore, a continuous spectrum be formed by suitable means and a 

 lithium-flame be placed between the source of light and the slit 

 of the apparatus, the spectrum is only affected in the place of 

 the lithium line, its brightness being increased in that part by 

 the radiation of the flame, while on the other hand it is diminished 

 by its power of absorption for waves of that particular length. 

 Suppose the absorptive power to be J. This would be the case, 

 according to the law we have demonstrated, if the brightness of 

 the line which constitutes the spectrum of the lithium-flame 

 were Jth of that of the corresponding line of the spectrum pro- 

 duced by a black body of the same temperature. The lithium- 

 flame would then be without effect on the spectrum produced 

 by any other source of light, provided the intensity of its 

 own spectrum were Jth of that of the corresponding line of the 

 spectrum produced in its absence. If the source of light were 

 proportionately brighter than this, the joint effect of it and the 

 lithium-flame would be to produce a comparatively dull line on 

 a bright ground ; and conversely, if the source of light were pro- 

 portionately duller, a bright line on a dull ground would become 

 visible. In the first case the apparent dullness of the line would 

 be greater in proportion to the brightness of the radiating body 

 behind the lithium-flame ; for in proportion as the light of the 

 former was increased, so would that of the latter become less 

 observable. For the particular value of the absorptive power 

 mentioned, the brightness of the lithium line can, however, never 

 be less than f ths of that of the surrounding parts of the spectrum. 



