v.] REVEKSAL OF LINES. 63 



identical in position with the bright band observed when the 

 molecules of the vapour of which the screen is composed radi- 

 ated light in the first instance. It is not, however, a case of 

 absolute blackness or absence of that particular ray, for the 

 molecules are set in vibration by the rays they absorb, and 

 therefore give out some light, but it is so feeble as to appear 

 black by contrast with the very much brighter rays coming 

 direct from the original source. 



The law which connects radiation with absorption and at once 

 enables us to read the riddle set by the sun and stars, is then 



FIG. 25. Diagram illustrating the graphical formulae employed on the opposite 

 page. Above is the continuous spectrum given by a source of white light. 

 Next below is the radiation spectrum (consisting of two bright lines) of sodium ; 

 and at the bottom is seen the effect of the interposition of the screen of sodium 

 vapour between the white light and the eye. The lines are still seen occupying 

 exactly the same positions as before, but they are reversed, i.e. they appear 

 black on a background of continuous spectrum. 



simply the law of sympathetic vibration. Kirchhoff s explana- 

 tion should now present no difficulty. The light orginally given 

 out from the sun is white, and would give a continuous spec- 

 trum, but before it reaches us it has to pass through a screen 

 consisting of the comparatively cool solar atmosphere ; and if 

 we imagine that atmosphere to contain sodium, iron, &c., among 

 the vapours, those vapours will behave just as our hypothetical 

 screen of molecules behaved ; they will abstract from the white 

 light passing through them those rays the rates of vibration 



