DrECEMBER 6, 1906| 
NATURE 139 
As a first simple motion we choose a vibration parallel | thread. 
to the lines of force. On the group of electrons which 
possess this motion the magnetic force has no influence ; 
the period, which we call T, remains unmodified. The 
other two simple motions are circular motions, clockwise 
or anti-clockwise, in planes perpendicular to the lines of | 
force. 
An electron performing either of these rotations will be 
acted on by a force which is directed towards or from the 
centre, dependent on the direction of the rotation. The 
magnetic field must, therefore, cause the speed of the 
electron either to increase or to decrease, and so will either 
diminish or increase the period. Therefore, instead of one 
motion with period T, we get under the influence of the 
field three motions with periods T, IT+v, T—v, uv being 
a small quantity. To each motion of the electrons there 
corresponds a luminous vibration, according to the electro- 
magnetic theory of light. Observing with a spectroscope 
we must, therefore, see each spectral line divided into 
three lines; each line becomes a triplet.’ 
I will show you a few examples of lines which are really 
divided into three components, in accordance with Lorentz’s 
theory (Fig. 2, iron; Fig. 3, part of ‘iron spectrum).° 
You will notice that each of the components remains very 
narrow; it is mot a hazy effect, but a very definite one. 
This certainly would not be the case if all molecules did 
not behave in the same manner, 
and if certain conditions of isotropy 
of the molecules were not fulfilled.* 
The consideration of the model 
may illustrate some other points 
which were foreseen by Lorentz’s 
theory. Consider the light emitted 
at a right angle to the lines of 
force. The three kinds of light seen 
in this direction are each due to 
vibrations of one kind, and_ there- 
fore polarised. We can, therefore, 
extinguish the light of the central 
component, or of the two external 
components, of the triplet by a 
Nicol. In one half of the slide 
shown the external components are 
extinguished ; in the other half the 
central one. So, for the first time, 
we were now able to get polarised 
radiations from the molecules of a 
gas. All attempts to produce such 
simple vibrations from gaseous 
molecules had hitherto failed. 
With some lines the central com- 
ponent and the outer ones differ 
much in intensity. If this be the 
case the spectroscope can be dis- 
pensed with entirely, and we may 
: observe a partial polarisation of the 
light emitted by the vapour in the field as found by 
Egoroff and Georgiewsky. We shall now consider the 
light emitted in the direction of the lines of force (Fig. 4). 
It is seen at once that each line must split up into two 
components. Moreover, both lines must be circularly 
polarised, but in opposite directions. With suitable 
arrangements; in one half of the field of view the one, in 
the other the second, component can now be extinguished. 
I observed this circular polarisation for the first time in 
the case of the sodium lines now shown. You see how 
complete the circular polarisation is. There is no trace of 
rectilinear or of elliptic polarisation.* 
When I first looked for this circular polarisation, I did 
not have the field of view divided into two parts, but the 
position of the line was determined by means of a spider’s 
1 Zeeman, Verslagen Kon Akademie v. Wetenschappen, Amsterdam 
Mei, Juni, October, 1897. Pil. Mag., July and September, 1897. 
2 The photographs illustrating this lecture are. excepting t! e diagrams, 
enlarged copies from negatives. The scale is different in the various cases. 
The separation of the outer components is of the order of one-sixth of the 
distance of the sodium lines (the vertical linesin Fig 5). No. 2 is a copy of 
one of the first photographs I obtained. The author is indebted to Prof. 
Runge for No. rz. The ronet is not distinctly shown in the latter repro- 
duction. In the Proceedings of the Royal. Institution some additional 
Figs. will be reproduced. 
% Lorentz, Annalen der Physik, Bd. 63, p. 278, 1807. 
4 Cf. Larmor, ‘‘ Aether and Matter,’’ p. 345, 1900. 
NO. 1936, VOL. 75] 
On the reversal of the magnetising current the 
luminous line moved. I do not wish to disguise the fact 
that no observation has ever afforded me so much pleasure 
as this one. 
It has already been remarked that we can also study 
the absorption lines which become visible when white 
— 
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Fic. 3. 
light is transmitted through the vapour. We then study 
the inverse effect. I shall use it to show you at least 
something directly depending upon the effect, because the 
effect itself is too young to appear before so large an 
audience. The inverse effect for light parallel to the lines 
of force plays a part in an experiment due to Righi.’ 
Consider a horizontal ray parallel to the axis of an electro- 
magnet with pierced poles, and let crossed Nicols be placed 
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Fic. 4. 
before and behind the instrument, as in Faraday’s experi- 
ment. A sodium flame in the field emitting two kinds of 
circularly polarised rays absorbs these same radiations, but 
does not stop the radiations polarised in the opposite 
1 Righi., C.2., cxxvil., p. 216, 1898. Nuovo 
Crm. (9), 8, p. 102, 1898. 
C.R., cxxviil., p. 45, 1899- 
