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



trum, seems to consist simply of lines of the same series so widened 

 out that they are blended into one mass. 



6. For the convenience of reference we have numbered the 

 lines from a conspicuous one which happens to fall between the 

 two D lines, nearer to the more refrangible one. There are 106 

 lines, counting from this line inclusive to a point somewhat 

 beyond b ; and we have measured the deviations of 31 of these 

 in a spectroscope giving a dispersion from A to H of about 24°. 

 We have from these measures deduced the inverse wave-lengths 

 by a comparison with the deviations of forty lines of iron, copper, 

 zinc*, and sodium, extending over the same range of the spec- 

 trum, of which the wave-lengths in air are recorded by Angstrom 

 and Thalen. The interpolation has been effected by a graphical 

 method ; and our measures in no case, when repeated, differed 

 by one minute of arc f. 



7. The first column of the following Table gives the numbers 

 of the several lines which we measured, reckoned from that line 

 which lies between the two D lines. The second column gives 

 the observed positions of the lines upon a scale of inverse wave- 



o 



lengths in air, viz. upon a scale of the reciprocals of Angstrom's 

 wave-lengths, which are wave-lengths in air of standard pressure 

 and 14° temperature. The third column contains the corre- 

 sponding positions upon a scale of inverse wave-lengths in vacuo, 

 obtained by applying to the numbers of column 2 the correc- 

 tions for the dispersion of air at 760 millims. pressure and 14 c 

 temperature, deduced from Ketteler's values J. The fourth co- 

 lumn gives the calculated positions, on the hypothesis that the 

 lines of the spectrum are equally spaced upon this last scale, as 

 they should be according to our theory. And the fifth column 

 gives the differences between columns 4 and 3, between the cal- 

 culated and observed positions. 



* The zinc-line to which Thalen assigns the wave-length 5/45, appears 

 rather to have a wave-length of about 5739. 



f The measuring-apparatus of the spectroscope has only recently been 

 completed. It is apparent to us that the instrument is capable of mea- 

 suring deviations even far more accurately than we have yet attempted. This 

 is due to the extraordinary precision with which Mr. Grubb's automatic ar- 

 rangement returns over and over again to the same position. It has, indeed, 

 almost made of the spectroscope a new physical instrument. 



t See Phil. Mag. vol. xxxii. (1866) p. 336. 



