Researches regarding the Laws of Chromatic Dispersion. 253 



Way's apparatus. Thus through his kindness I probably had 

 the advantage of examining a purer as well as a far more bril- 

 liant mercurial light than any previous observer. 



Attention is being particularly directed at the present time to 

 the fact that the luminous bands of some artificial flames coin- 

 cide exactly with dark lines in the solar spectrum, and that cer- 

 tain flames absorb light of the same refrangibility as they emit. 

 With reference to the latter phenomenon, I have looked at 

 the double spectrum of the sun shining through the mercury 

 flame; but the rays from the metal were too intense to permit 

 of any evidence of absorption of the solar beams in the same 

 place. A glance at the preceding Table will show that a bright 

 line occurs in the spark of mercury as in many other artificial 

 lights just where D occurs in the solar spectrum ; another ray- 

 appears to have the same refrangibility as the most prominent 

 line between G and FT, namely G 33 : otherwise there seems to 

 be no coincidence. 



XXXI. Further Researches regarding the Laws of Chromatic Dis- 

 persion. By Mungo Ponton, F.R.S.E* 



THROUGH the kindness of Dr. J. H. Gladstone, the author 

 has been recently furnished with additional experimental 

 data by which to test the laws developed in his former commu- 

 nication. 



He has also revised his method of determining the relative 

 values of the wave-lengths corresponding to the seven principal 

 fixed lines of the spectrum, referred to that of B as unity. 



Calling E^H=/>, B-f-E = <r, C-kE = t, Eh-G=u, D-^-E = % , 

 and E-r-F =i|r, the relations subsisting among these six fractions, 

 as fairly deducible from Fraunhofer's two sets of observations, 

 may be expressed by the following six equations : — 



(1) r-u=0-022'. 



(2) /3 = 60(r-v) = l-33 / . 



(3) 3( % -^) = 5(r-t,) = 0-ll'. 



(4) (o- + x) — (t + v) = 2(t-i;) =0-044'. 



(5) 2(o-- % ) = 17(t-u) =0-377'. 



(6) 2(o- + t + i; + x)=441(t-u) = 9'799'. 

 Hence the values of the fractions are— 



* Communicated by the Author. 



