Page:Proceedings of the Royal Society of London Vol 60.djvu/549

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I ask permission to bring before tbe 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 broadened 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 deception 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, 7.e., a fact connected with the interchange of eneigy 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 disappear; 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, 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