Mechanics of Luminosity. 265 



tinum wire to that of the sodium flame was almost the same 

 as in the previous experiments in which the wire was heated 

 by a current. There is no object in giving the particular 

 values here. 



In exactly the same way experiments were made with solu- 

 tion of strontium chloride, which yields a spectrum of bands, 

 solutions equivalent to the sodium chloride solution being 

 scattered in the flame. The result obtained was that the 

 total brightness is of the same order as that of the sodium 

 flame, which also may be inferred from the strong colora- 

 tion of the flame. 



Here, therefore, nearly the same total energy is distributed 

 over a series of bands. 



32. We will now determine the ratio of the true coefficients 

 of emission of two sources of light. For this purpose let 

 us consider the following points: — 



If we have a narrow spectral line whose boundaries are at 

 X 1 and X 2 , and if we examine this with a spectral-photometer 

 of small dispersion, it appears in the spectrum of the same 

 width as the slit. Let the dispersion be so chosen that the 

 edges of the slit correspond to wave-lengths X and X + A in 

 the spectrum. If then, by means of a spectrophotometer, 

 we compare the brightness of the spectrum -line X and of a 

 continuous spectrum, then each point in the image of the 

 slit, corresponding to the line-spectrum, receives rays between 

 \ and X 2 , an( i m the image of the slit corresponding to the 

 continuous spectrum each point receives rays between 

 X and X + A. If the intensities corresponding to the wave- 

 length X are in the two cases i f and i" respectively, then the 

 total intensities Jx and J 2 in the first and second image 

 respectively are : — 



J x = I i'dX and J 2 = I i"dX. 

 Ja x J\ 



The quantities i' and i" are proportional to the true coefficients 

 of emission ; the ratio of the intensities is therefore 



r\ 2 /*a+a 



V= i'dX : i"dX. 



In the simplest case we may assume that i' and i" are con- 

 stants, i. e. that the spectral lines between X 1 and X 2 , as also 

 the continuous spectrum between X and X + A, possess every- 

 where the same brightness, then 



•y ^ X 2 — Xj 



y ~F'~ A - * 

 The observed ratio of intensities is therefore proportional to 



