338 



Mr. H. G. Madan on the Reversal of 



been obtained by calculating strictly according to the theoretical 

 formula (0). According to this principle, it is only necessary 

 to apply these differences to the values of ~k previously calcu- 

 lated from Eraunhofer's data, to obtain the values that would 



o 



be given by Angstrom's data. These calculations having been 

 gone through, the comparison, to the third place of decimals, 

 of the observed and calculated values of X for the two sets of 

 data stands as follows : — 







Excess of calculation. 





Excess of calculation. 



Ray. 



Xby 



Fraunhofer. 







o Vbfc 



Angstrom. 







Flint-glass 



Oil.of 



Flint-glass 



Oil of 







Ho. 13. 



Cassia. 





No. 13. 



Cassia. 



B 



2-541 



0-000 



0-000 



2-5397 



0000 



0000 



C 



2-422 



4-0003 



4-0-006 



2-4263 



-0-002 



4-0001 



D 



2- 175 



-0-001 



-0001 



2-1786* 



-0 003 



-0 003 



E 



1-945 



0000 



000 



1-9484 



0-000 



0000 



F 



1-794 



4-0-002 



-0-003 



1-7973 



4-0003 



-0001 



G 



1-587 



4-0-005 



-0-004 



1-5923 



4-0 004 



-0003 



H 



1-464 



0-000 



0-000 



1-4672 



0000 



o-ooo 



It appears from this comparison that the excesses of calculation 



o 



are somewhat smaller w T ith Angstrom's values than with Fraun- 

 hofer's, especially in the case of oil of cassia, the more refractive 

 substance. 



With this communication I conclude the series of arguments 

 by which I maintain that the Undulatory Theory of Light rests 

 legitimately on no other than a hydrodynamical basis. 



Cambridge, April 22, 1865. 



XL VII. On the Reversal of the Spectra of Metallic Vapours. 

 By E.G. Madan, F.C.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, April 11, 1865. 



/|~AY I be allowed to mention a simple and convenient 



r JL method of illustrating one of the most important points 

 in Bunsen and KirchhofPs spectrum discoveries, viz. the reversal 

 of the spectra of metallic vapours ; the most familiar example of 

 which is the reversal of the sodium-line D ? 



I have tried most of the various methods proposed for effect- 

 ing this object, but none have appeared to me so easy and 

 effective as the following. 



* This value applies to the middle of the double line. 



