42 REPORT — 1871. 



seven metres per second produced a very sensible effect on the velocity -with which 

 light was propagated in the direction of the motion ; in other words, when the 

 molecular motions had a preponderance in one direction, this was found to alter 

 the refi-active index in that direction. This shows that the molecular motions do 

 affect the refractive index ; and it is perhaps not too much to presume that the 

 phenomena of the irrationality of the spectra produced by prisms of different mate- 

 rials of double refi'action and polarization in crystals of^ other than the cubical 

 system, and of circular polarization in solids and liquids, will be found to residt 

 from modifications of the irregular motions either of or within the molecides. 

 Other facts appear to confirm this presumption : where from the form of a crystal 

 we have reason to suppose that the irregular molecular motions are not symme- 

 trically distributed in different directions, there we uniformly find the phenomena 

 of double refraction ; and in those solids where they are symmetrically disposed 

 the refraction becomes double if they are exposed to strain, i. e. as soon as an 

 unsymmetrical distribution of the molecular motions is artificially induced. 



On the whole we appear j ustified in drawing the probable inference that all the 

 phenomena of transparency are intimately associated witli the molecular motions 

 which want that kind of" regularity which would fit them to be the source of 

 luminous undulations. What is certain is, first, that certain periodic molecidar 

 motions do produce the phenomena of opacity in gases ; and secondly, that iiTe- 

 gular molecular motions are incapable of producing the effect of opacity, since they 

 cannot radiate. By irregular motions, where the phrase occurs in this communi- 

 cation, are to be understood motions which are not approximately periodic, or 

 which from any other cause cannot set up in the aether such an imdulation as that 

 which constitutes radiant heat. 



On the advantage of referring the positions of Lines in the Spectrum to a Scale 



of Wave-numbers. By G. Johnstone Stoney, M.A., F.R.S. 

 _ At the last Meeting of the British Association Mr. Stoney made a communica- 

 tion, from which it seemed to appear that each periodic motion in the mole- 

 cules of a gas will in general (i.e. unless the motion be a simple pendulous 

 one, or else mechanically small) give rise to several lines in the spectrum of the 

 gas, and that the lines which thus result from one motion have periods that are 

 harmonics of the periodic time of the parent motion. Since tliat time he has been 

 engaged, in conjunction with Dr. Emerson Reynolds, of Dublin, in testing this 

 theory ; and in this inquiry it has been found convenient to refer the positions of 

 all lines in the spectrum to a scale of reciprocals of the wave-lengths. This scale 

 has the great convenience, for the purposes of the investigation, that a system of 

 lines with periodic times that are harmonics of one periodic time are eqiiidistant 

 upon it ; and it has the fm'ther convenience, which recommends it for general use, 

 that it resembles the spectrum as seen in the spectroscope much more closely than 

 the scale of direct wave-lengths used by Angstrom in his classic map. 



The position marked 2000 upon this scale occurs about the middle of the 

 spectrum, and corresponds to Angstrom's wave-length 5000. The numbers which 

 Angstrom uses are tenth-metres, i. e. the lengths obtained by dividing the metre 

 into 10'° parts ; and from this it follows that each number on the new scale 

 signifies the number of light-waves in a millimetre : thus 2000 upon a map drawn 

 to this scale marks the position of the ray whose wave-length is ^^^ of a milli- 

 metre. The new scale may therefore be appropriately called a" scale of wave- 

 numbers. If, then, Jc be the wave-number of a fundamental motion in the aether, 



its wave-length will be- th of a millimetre, and its harmonics will have the wave- 



lengths — , — &c. ; in other words, they occupy the positions 2k, Bk, &c. upon 



the new map. Hence it is easy to see that a system of lines which are equally 

 spaced along the map at intervals of k divisions are harmonics of a fundamental 



motion whose wave-number is k, whose wave-length is -th of a millimetre, and 



