Absorbing Powers of different Bodies for Light and Heat. 1 7 



lines D, and if rays from some other source of light be supposed 

 to pass through this flame, and then to be separated into a 

 spectrum, the brightness of the part of the spectrum corre- 

 sponding to the lines D depends on the radiation of the flame 

 alone. If, then, another sodium-flame of the same temperature 

 be interposed, the spectrum remains unaltered ; but if the flame 

 interposed be of lower temperature, the lines must become duller. 

 The effect on the solar spectrum of a gas-flame containing sodium 

 is in this way accounted for, if it be admitted that the tempera- 

 ture of such a flame is lower than that of the outermost envelope 

 of the sun's atmosphere ; and this must certainly be the case, 

 since the external portion of the sun's atmosphere cannot have 

 a lower temperature than that of the focus of a powerful concave 

 mirror directed towards the sun. 



What has been stated concerning sodium is equally true of 

 every other substance which, when placed in a flame of any sort, 

 produces bright lines in its spectrum. If these lines coincide 

 with the dark lines of the solar spectrum, the presence in the 

 sun's atmosphere of the substances which produce them must 

 be concluded, provided always that the lines in question cannot 

 have their origin in the atmosphere of the earth. In this way 

 means are afforded of determining the chemical constitution of the 

 sun's atmosphere; and the same method even promises some 

 information concerning the constitution of the brighter fixed 

 stars*. 



§ 15. From the proposition demonstrated in the first part of 

 this essay, it follows that a body that absorbs more rays polarized 

 in one plane than in another, must emit proportionately more 

 rays of the first description than of the latter. Whence, as is 

 known to be the case, a red-hot opake body with a smooth 

 surface must emit rays in directions oblique to this surface partly 

 polarized perpendicularly to the plane passing through the ray 

 and normal to the surface ; for of the incident rays polarized 

 perpendicularly to the plane of incidence, the body reflects less 

 and absorbs more than of the rays whose plane of polarization 

 is the plane of incidence. By means of this principle the state 

 of polarization of the emitted rays can easily be determined when 

 the law of reflexion of the incident rays is known. 



A tourmaline plate split perpendicularly to its optic axis 

 absorbs at ordinary temperatures more of the perpendicularly 



* In two communications laid before the Berlin Academy of Sciences 

 on the 27th of October and the 15th of December, 1859, some statements 

 are to be found concerning the physical constitution of the sun's atmo- 

 sphere which are not introduced here. In the second of those communica- 

 tions also the proposition that forms the principal subject of this essay is 

 proved in another way, but with less generality. 



Phil Mag. S. 4. Vol. 20. No. 130. July 1860. C 



