298 Lord Rayleigh on the Limit to Interference 



was adjusted in the position where the strongest effect was ob- 

 served, and withdrawn from the prism along a radius of the 

 circle, the sparks could be recognized up to a distance of 5 or 

 6 metres. A screen placed in front of or behind the prism 

 always quenched the sparks ; a proof that the action occurred 

 in fact through the prism, and did not reach the secondary 

 conductor by any other part. The experiments were repeated 

 after placing the focal lines horizontal, but without altering 

 the position of the prism. No alteration in the phenomena 

 produced was observed. A refracting angle of 30° and 

 a deviation of 22° in the neighbourhood of minimum devia- 

 tion corresponds to a refractive index of 1'69. The refractive 

 index for light is given for pitch-like substances between 1*5 

 and 1*6. The uncertainty of our determination and the im- 

 purity of the material employed does not permit of our 

 assigning greater importance to the magnitude or signification 

 of this difference. 



We have represented the phenomena investigated by us as 

 rays of electric force. We may in conclusion perhaps regard 

 them as light-rays of very great wave-length. To me at least 

 the experiments described seem eminently fitted to remove all 

 doubt as to the identity of light, radiant heat, ard electrody- 

 namic wave-motion. I believe that we shall now with more 

 confidence avail ourselves of the advantages which the assump- 

 tion of this identity offers, both in the domains of optics and 

 of electricity. 



XXXV. On the Limit to Interference when Light is radiated 

 from Moving Molecules. By Lord Rayleigh*. 



IN a recent number of Wiedemann's Annalen, Ebertf 

 discusses the application of Doppler^s principle to the 

 radiation from the moving molecules of an incandescent gas f , 

 and arrives at the conclusion that the widths of the spectral 

 lines, as calculated upon the basis of the principle, are much 

 greater than is consistent with experiments upon interference 

 with a large reladve retardation. This is a matter of no small 

 importance. Unless the discrepancy can be explained, the 

 dynamical theory of gases would, it appears to me, have received 

 a heavy blow, from which it could with difficulty recover. If 

 it be true that a gas consists of molecules in irregular motion, 



* Communicated by the Author, 

 t Wied. Ann. xxxyI. p. 466 (1889). 



X Lippich, Pogg. Ann. cxxxix. p. 465 (1870). Rayleigh, 'Nature,' 

 vui. p. 474 (1873). 



