42 Messrs. G. J. Stoney and J. E. Reynolds on the Cause 



we looked at several of the absorption-spectra of coloured vapours, 

 to see whether amongst them we could find one in which there 

 is a system of lines which we might hope to refer to a single 

 motion in the molecules of the vapour, free from lines emanating 

 from other motions in the molecules of the vapour, and suffi- 

 ciently separated from one another to be easily measured. A 

 few days after we commenced operations we were so fortunate 

 as to meet with the object of our search. The brown vapours of 

 chlorochromic anhydride (Cr0 2 Cl 2 ) when interposed between 

 the lime-light and the spectroscope gave a spectrum of the re- 

 quisite simplicity. 



4. In order to test that part of the theory which indicates that 

 the periodic times of the wave-vibrations of the several lines are 

 harmonics of one periodic time, we find it convenient to refer 

 the positions of all lines to a scale of reciprocals of the wave- 

 lengths. This scale of inverse wave-lengths has the great conve- 

 nience for our present purpose, that a system of lines with 

 periodic times that are harmonics of one periodic time will be 

 equidistant upon it : it has also the minor convenience that it 

 much more closely resembles the spectrum,as seen, than the scale 

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

 Upon our scale the inverse wave-length 2000 corresponds to 

 Angstrom's direct wave-length 5000. The numbers which 



o 



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

 dividing the metre into 10 10 parts; and from this it follows 

 that each number upon our scale is the number of light- waves 

 in a millimetre : thus 2000 upon our scale means that the corre- 

 sponding wave-length is 2 oVo °^ a m ^ nmetre » Now, if k be the 

 inverse wave-length, expressed upon our scale, of a fundamental 



motion in the sether, its direct wave-length will be -7th of a milli- 

 metre, and its harmonics will have the wave-lengths — , — , &c. 



Aii ok 



Accordingly the inverse wave-length of the rcth harmonic will be 



i n =(n + l).k (I) 



Hence it is easy to see that a system of harmonics which are 

 equally spaced along our scale at intervals of k divisions are har- 

 monics of a fundamental motion whose inverse wave-length is k } 



whose direct wave-length is -rth of a millimetre, and whose pe- 



riodic time is p where t is the periodic time of an undulation in 

 the sether consisting of waves one millimetre long. If we use 



