101 



CHAP. XL EVIDENCE AFFORDED BY THE SHIFTING OF LINES. 



RECENT work in America, by means of the great dispersion afforded by 

 Rowland's concave gratings, has supplied us with results* 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 exami- 

 nation of the Rowland series of photographs of the solar and metallic 

 spectra taken by means of a concave grating of 21 \ feet radius and 

 20,000 lines to the inch an instrument of research which, so far as 

 my own experience goes, is obtained with great difficulty by workers 

 in this country. 



Mr. Jewell's investigations began in 1890. Messrs. Humphreys and 

 Mohler studied in 1895 the effects of pressure on the arc spectra of the 

 elements, work suggested by Mr. Jewell's prior researches. 



Mr. Jewell, as a basis for his new conclusions, investigated under 

 modern conditions classes of phenomena which I was the first to- 

 observe and describe more than 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 an idea of the meaning of some of the terms employed. 



I first employed, as stated on p. 22, 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. 



(1) 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 

 name 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. Trans., 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.f 



* Astropbysical Journal, February, 1896, vol. iii, p. 111. 

 f Chemistry of the Sun, p. 379. 



