Mr. Talbot on the Nature of Light. 1 1 7 



ment that only the violet rays are effective in producing phos- 

 phorescence. This is confirmed by the recent experiments 

 of Osann. The violet rays also cause the discolouration of 

 the nitrate of silver, the rest of the spectrum being nearly in- 

 effective. This analogy is much in favour of my argument. 



§ 8. I come now to consider what probable conclusions 

 can be drawn respecting the nature of light from the nu- 

 merous absorptions which it exhibits in passing through ni- 

 trous gas, and the other coloured gases. For the discovery 

 of this most remarkable phenomenon it is well known that 

 science is indebted to Sir David Brewster. The absorptions 

 or dark lines in the spectrum produced by iodine vapour, are 

 described by Professor Miller*, as being about a hundred in 

 number, and equidistant. I have, however, found by careful 

 observations that they are not equidistant, but that they be- 

 come gradually more crowded towards the blue end of the 

 spectrum. Baron von Wredef has observed the same fact, 

 and established it by actual measurement. 



This approximation of the lines takes place gradually, and 

 seems a consequence of some simple general law. There is 

 also a space at each extremity of the spectrum in which no 

 lines are visible. Now, in the former part of this paper I have 

 advanced the hypothesis that the vibrations of light and those 

 of material molecules are capable of mutually influencing each 

 other. It remains to be seen, whether the same hypothesis 

 does not afford a clue to the explanation of this apparently 

 complex phenomenon of absorption. 



It is known that certain gases combine rapidly when ex- 

 posed to sunshine, which do not unite in the dark : no doubt 

 because the light causes the molecules to vibrate, so as to 

 come within the sphere of each other's attraction. 



I conclude from this, and many other facts of the same 

 kind, that light when traversing a transparent medium is able 

 to excite motion among its particles. This being admitted, 

 let us suppose iodine vapour so constituted that its molecules 

 are disposed to vibrate with a rapidity not altogether dissi- 

 milar to that of light. Now, if the differently coloured rays 

 differ also (as is probable) in rapidity of vibration, some of 

 them will vibrate in accordance, and others in discordance, with 

 the vibrations of the iodine gas. And these accordances and 

 discordances will succeed each other in regular order, from 

 the red end of the spectrum to the violet end ; each discord- 

 ance being marked by a dark line or deficiency in the spectrum, 

 because the corresponding ray is not able to vibrate through 

 the medium, but is arrested by it and absorbed. I communi- 



• Lond. and Edinb. Phil. Mag., vol. ii. p. 381. 

 t Poggendotf's Anna/s, vol. iii. p. 353. 



