March 5, 1896] 



NATURE 



415 



THE SHIFTING OF SPECTRAL LINES. 



THE Astrophysical Journal for February contains 

 some papers of the highest interest, touching small 

 variations in the wave-lengths of spectral lines and the 

 causes which produce them. These are stated to have 

 been, in the first instance, established by Mr. Jewell by 

 an examination of the Rowland series of photographs of 

 the solar and metallic spectra taken by means of a con- 

 cave grating of 2ii feet radius and 20,000 lines to the 

 inch— an instrument of research which, so far as my own 

 experience goes, is not to be obtained by workers in this 

 country. 



Mr. Jewell's investigations began m 1890. Another 

 paper by Messrs. Humphreys and Mohler details the 

 results of work begun last year on the effects of pres- 

 sure on the arc spectra of the elements, work suggested 

 by Mr. Jewell's prior researches. 



Mr. Jewell has, as a basis for his new conclusions, 

 practically studied under modern conditions classes of 

 phenomena which I was the first to observe and de- 

 scribe, as near as may be a quarter of a century ago. 



To show the relation of the new work to the old, it 

 is best to begin with a short historical statement, which 

 will have the advantage of giving to non-experts an 

 idea of the meaning of some of the terms employed. 



I first employed the method of throwing an image of a 

 light source on to the slit of a spectroscope by means of 

 a lens in 1869, and some of the results obtained by the 

 new method were the following. 



(i) The spectral lines obtained by using such a light 

 source as the electric arc, were of different lengths ; some 

 only appeared in the spectrum of the core of the arc, 

 others extended far away into the flame and outer 

 envelopes. This effect was best studied by throwing the 

 image of a horizontal arc on a vertical slit. The lengths 

 of the lines photographed in the electric arc of many 

 metallic elements were tabulated and published in 

 Phil. Trails., 1873 and 1874. 



(2) The longest lines of each metal generally were wider 

 than the others, the edges fading off; and they reversed 

 themselves ; by which, I mean, that an absorption line 

 ran down the centres of the bright lines. These results 

 were afterwards confirmed and extended by Cornu 

 ("Chemistry of the Sun," p. 379). 



(3) From experiments with mixtures of metallic 

 vapours and gases, it came out that the longest lines of 

 the smaller constituent remained visible after the shorter 

 lines had disappeared, the spectrum of each substance 

 present getting gradually simpler as its percentage was 

 reduced,! the shorter lines being extinguished gradually. 

 Shortly after these observations were made, I included 

 among some general propositions:^ "In encounters of 

 dissimilar molecules the vibrations of each are damped." 



(4) The various widths of the lines, especially of the 

 winged longest ones, were found to depend upon pres- 

 sure or density, and not temperature. ^ 



(5) The " longest lines " of any one metal were found 

 to vary in their behaviour in most extraordinary fashion in 

 solar phenomena, being furthermore differentiated from 

 the shorter ones ; and on this and other evidence I founded 

 my working hypothesis of the dissociation of the 

 chemical elements at the solar temperature. In 1876 I 

 set out the facts with regard to calcium. 



(6) In 1883, Prof. W. \'ogel, in a friendly criticism, 

 pointed out the evidence then beginning to accumulate, 

 that under certain circumstances the wave-lengths of 

 lines are changed.* In 1887, I extended this evidence,^' 



1 /•////. Trans., 1873, p. 482- 



- " .Studies in Spectrum Analysis," 1S78, p. 140- 



'^ Phil. Trans., 1872, p. 253. 



■* Nature, vol. xxvii., 1883, p. 233. 



5 " Chemistry of the Sun," p. 369. 



and I think it was I who coined the word " shift " to 

 express these changes.^ 



I now pass on first to the results which Mr. Jewell 

 claims to have established. 



With the enormous dispersion produced by the 

 instruments referred to, it is found that certain metallic 

 lines, but not all, are displaced or " shifted " towards the 

 violet when compared with the corresponding solar 

 lines. " There was a distinct difference in the displace- 

 ment, not only for the lines of different elements, but also 

 for the lines of different character belonging to the same 

 element." 



The " different character " above referred to turns out 

 to relate not so much to the intensity as to the 

 length, and, associated with this, the reversibility of the 

 lines ; the longest lines are the most displaced, the 

 shortest, least. 



Further, in the spectrum of the arc itself, the position 

 of a line with but little material present " was approxi- 

 mately the same as the position of the line when 

 reversed." Now since the longest lines are most 

 displaced to the violet, this means that the smaller the 

 quantity of a substance present the greater is the dis- 

 placement towards the violet ; and therefore the greater 

 the quantity present, the greater the displacement towards 

 the red. 



Further on, Mr. Jewell expressly states it was found 

 that with an increase in the amount of the material in 

 the arc there was an increasing displacement of the line 

 towards the red," and then he adds, "unless the hne 

 became reversed, when all further progress in that 

 direction ceased." 



Here is an observation regarding the red line ot 

 cadmium. " It was found that if the micrometer wires 

 were set upon it with very little cadmium in the arc, then 

 as the amount was increased the line almost bodily left 

 the cross-hairs, always moving towards the red." 



Mr. Jewell considers he has established that the 

 vibration period of an atom depends to some extent 

 upon ics environments. " An increase of the density of 

 the material, and presumably an increase of pressure, 

 seemed to produce a damping effect upon the vibration 

 period." • j j 



My result of 1872 with regard to pressure is endorsed, 

 " the new results are found to be due to pressure and 



not temperature." 



We seem then now to be in presence of two damping 

 effects in the case even of metallic lines, one which 

 extinguishes Hnes when we deal with dissimilar molecules, 

 and one which changes their wave-length towards the 

 red when we deal with similar molecules. 



A carefully prepared table is given by Mr. Jewell, 

 showing the origin, intensity and character of the solar 

 Hnes considered, the intensity and character of the cor- 

 responding metallic lines, the wave-lengths of both, and 

 the observed displacement. 



There are many references to solar phenomena in Mr. 

 Jewell's paper, but I do not propose to discuss them now. 

 There is one point, however, I must refer to, in justice to 

 my critics. He considers that the conclusions to be 

 drawn from a study of the new shifts "effectually 

 disposes of the necessity of any dissociation hypothesis 

 to account for most solar phenomena." 1 have already 

 pointed out that this was Prof. W. X'ogel's conclusion 

 with regard to possible shifts, so far back as 1883. 



It is quite easy. "Two adjacent lines of iron, for 

 instance, may show the effects of a violent motion of iron 

 vapour in opposite directions, in the neighbourhood of 

 spots, or one line (the smaller one corresponding to one 

 of Lockyer's ' short lines ') may show a broadening and 

 increase of intensity in the spectrum of a sun-spot, 



1 Since the parentage is uncertain, I may say that perhaps "shiftings" 

 would have been a better word, as shift is otherwise employed, eg. I^ve s 

 last shift (translated by a French author, la demiire chcm,se de I amour). 



NO. 1375. VOL. 53] 



