[ 376 ] 



XLV. On the Mechanics of Luminosity. 

 By E. Wiedemann. 



[Concluded from p. 267.] 



General Considerations as to the Coefficients of True and 

 Total Emission. 



33. TN connexion with these investigations, before passing 

 JL to the calculation of the store of luminous energy, I 

 wish to consider certain cases, in which the difference be- 

 tween the total and true coefficients of emission, or the bright- 

 nesses corresponding to them in the visible spectrum, must be 

 taken into account. The difference between true and total 

 brightness is clearly seen in a comparison of the band-spectra, 

 such as are produced by the haloid compounds of the metals of 

 the alkaline earths, and the line-spectra as shown by the alkaline 

 metals themselves. The total emission in the band- and line- 

 spectra is, as mentioned above, nearly the same ; but the true 

 emission of the line-spectra is much greater than that of the 

 band-spectra, for with a dispersion which spreads out the bands 

 to continuous relatively-feeble broad bands, the lines are still 

 sharp and bright. If in the same spectrum there are bright 

 lines and less bright lines, it does not follow that the latter 

 correspond throughout to a smaller true emission if the brighter 

 ones are not also the narrower. These two circumstances must 

 be taken into account, for example, in the reversal of lines, 

 since the absorption corresponds to the true emission. 



A comparison of the brightness of lines in the spectrum, 

 for which the dispersion does not extend so far as to expand 

 them, can teach us little as to the true coefficient of emission, 

 and the kinetic processes corresponding therewith. At most 

 thus much, that at one point of the spectrum there exists a 

 greater energy of motion than at another. This holds good, 

 for example, for the investigations of M. Lagarde* on the 

 total brightness of the hydrogen-lines. The increase in 

 brightness in the separate lines produced by stronger dis- 

 charges may be explained just as well by an expansion of the 

 lines — that is, by the addition of new vibrations to the original 

 ones — as by an increase in the true brightness, that is, by an 

 increase in the energy of the original vibrations. 



Determination of the Store of Luminous Energy. 

 34. We will now endeavour to determine the store of lumi- 

 nous energy L, upon which the luminous motions depend, 



* Ann. de Chim. et de Phys. [6] iv. p. 248 (1885). 



