Prof. U. Lodge. Radiation Frequency. 513 



" The Influence of a Magnetic Field on Radiation Frequency." 

 Communication from Professor OLIVER LODGE, F.R.S. 

 Received and read February 11, 1897. 



I ask permission to bring before the notice of the Fellows a 

 notable discovery recently made at Leyden by Dr. P. Zeeman, who is 

 now elected Professor of Physics in the University of Amsterdam. 

 To put myself in order, I will state that I have set up apparatus 

 suitable for showing the effect, and have verified its primary feature, 

 viz., that both lines in the ordinary spectrum of sodium are broad- 

 ened when a magnetic field is concentrated upon the flame emitting 

 the light. 



Zeeman has observed it likewise with lithium, and with absorption 

 as well as with emission spectra ; taking precautions against decep- 

 tion by spurious effects due to changes of density or of temperature. 

 It is thus probably not a chemical fact, dependent on the nature of a 

 substance, but a physical fact, dependent on the nature of radiation 

 and absorption, i.e., a fact connected with the interchange of energy 

 between ether and matter. 



Faraday appears to have looked for some such phenomenon in the 

 course of his latest magneto-optic researches in 1862, but he had not 

 a Rowland concave grating at his disposal, and the effect is small. 



I saw it with a 1-inch flat reflection grating containing 14,600 

 lines, and with an oxy-coal gas flame playing on pipe clay supporting 

 carbonate of soda between pointed poles. I tried to see it by 

 widening the slit till the D lines almost encroached on each other ; 

 thinking thereby to see the residual dark space obliterated by the 

 magnetic action. A luminous haze seemed to spread over the dark 

 chink when the magnet was excited, but the chink itself did not dis- 

 appear ; and the effect is more conspicuous and easier to observe 

 when the narrowest slit possible is used, and when a micrometer 

 spider-line is set down the middle of one of the D lines, of the second 

 order spectrum, well defined in a field of considerable magnifying 

 power. 



The broadening is then unmistakable, and is symmetrical on each 

 side; but I judge that the edges are not so bright as the central 

 portion. The line appears brightened as well as broadened, i.e., the 

 previous borders of the line are brightened, and there are also 

 gradated extensions. If the focussing is sharp enough to show a 

 narrow, dark reversal line down the middle of either sodium line, that 

 dark line completely disappears when the magnet is excited. 



With the help of Professor H. A. Lorentz, the discoverer has 

 initiated a simple theory of the effect, by considering the effect of 



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