109 



CHAP. XII. EVIDENCE AFFORDED BY THE MAGNETIC PERTURBA- 

 TIONS OF LINES. 



LONG before the present electro-magnetic theory of light was formu- 

 lated in its present shape, several observers endeavoured to see if any 

 spectrum change was to be noted when the light source was placed in 

 a strong magnetic field. 



Of these, Professor Tait seems to have been the earliest. He made 

 the attempt in 1855 :* it led to no result. The same thing happened to 

 Faraday in 1862. Indeed, his experiment on this question was the 

 last he ever made. I extract the following account of it from his life 

 by Dr. Bence Jones : t 



" 1862 was the last year of experimental research. Steinheil's appa- 

 ratus for producing the spectrum of different substances gave a new 

 method by which the action of magnetic poles upon light could be 

 tried. In January he made himself familiar with the apparatus, and 

 then he tried the action of the great magnet on the spectrum of chloride 

 of sodium, chloride of barium, chloride of strontium, and chloride of 

 lithium." 



An experiment made on March 12 is thus recorded : 



" The colourless gas flame ascended between the poles of the magnet, 

 and the salts of sodium lithium were used to give colour. A Nicol's 

 polarizer was placed just before the intense magnetic field, and an 

 analyzer at the other extreme of the apparatus. Then the electro- 

 magnet was made and unmade, but not the slightest trace of effect on 

 or change in the lines in the spectrum was observed in any position of 

 polarizer or analyzer. 



" Two other pierced poles were adjusted at the magnet, the coloured 

 flame established between them, and only that ray taken up by the 

 optic apparatus which came to it along the axis of the poles, i.e., in the 

 magnetic axis, or line of magnetic force. Then the electro-magnet was 

 excited and rendered neutral, but not the slightest effect on the polar- 

 ized or unpolarized ray was observed." 



About the year 1872, Professor Clifford and myself made some 

 experiments with the large Steinheil spectroscope then in use in my 

 laboratory at the School of Science ; the only magnet available was a 



* Proc. Roy. Soc. Edin., vol. ix, p. 118, 1875-6. 

 t Vol. ii, p. 449, 1870. 



