176 
WATOLRE 
[JUNE 22, 1899 
possess no permanent relation to any direction around 
the ray, so that if the magnetic action should happen to 
be a twisting of the vibrations round the ray, it will be 
impossible to detect this twist in the case of ordinary 
light. 
The Faraday Effect. 
As a matter of fact it is a twist of this kind that 
actually happens, and this is probably what Faraday 
anticipated. In order to detect it, therefore, it is necessary 
to employ a beam of light in which the vibrations are 
restricted to a single plane passing through the ray. 
Such light is said to be plane polarised, and may be 
obtained by transmitting common light through a doubly 
refracting crystal. Faraday found that when a beam of 
this plane polarised light is passed through the magnetic 
field, in the direction of the lines of force, a distinct 
effect takes place, and that the effect is a twisting of the 
plane of polarisation of the light vibrations as they pass 
through the magnetic field, or, to be more precise, as the 
light passes through the matter occupying the field. 
This is the Faraday effect. Its magnitude depends on 
the strength of the field, and upon the nature of the 
matter through which the light passes in that field. 
This latter is an important fact that should not be lost 
sight of in reasoning upon the nature of this effect. The 
presence of matter in the field appears to be necessary. 
The effect is not observed in a vacuum, but becomes 
greater as the field becomes filled with matter of greater 
density. It is therefore not a direct action of the 
magnetic field on the light vibrations, but rather an 
indirect action exerted through the intervention of the 
matter which occupies the magnetic field. 
This action, as we have said, is a rotation of the plane 
of polarisation of the beam of light, and it arises from 
the circumstance that in passing through the magnetic 
field vibrations which take place from right to left do not 
travel forward with the same velocity as those which take 
place from left to right. There is no change in the 
periods of the vibrations, it is essentially a change of 
velocity of propagation that occurs. If we examine the 
transmitted light with a spectroscope, we find that the 
wave-lengths are unaltered, but that the amount of 
rotation of the plane of polarisation is different for waves 
of different lengths. The law which governs the effect 
is that the rotation of the plane of polarisation varies 
inversely as the square of the wave-length of the light 
employed. 
Second Form of Experiment (Faraday and Fievez). 
You will have noticed that in the foregoing experiment 
the source of light was placed quite outside the field of 
magnetic force, while the beam of light was transmitted 
through the field for examination. Now we might place 
the source of light itself in the magnetic field, and then 
examine if the light emitted by it is in any way affected 
by the magnetic force. This variation of the experiment 
suggests itself at once, and was indeed also tried by 
Faraday—ain fact, it formed his last experimental research 
of 1862, but without success. The same experiment has 
been tried, no doubt, by many other physicists with the 
same negative result. 
The first recorded success, or at least partial success, 
was by M. Fievez in 1885. He placed the source of 
light—a gas flame impregnated with sodium vapour-— 
between the pole-pieces of a powerful electro-magnet. 
This being done, the light radiated by the flame was 
passed through the slit of a highly dispersive spectroscope 
and examined. What M. Fievez observed was that the 
bright spectral lines became broadened by the action of 
the magnetic field on the radiating source. His account 
is, perhaps, somewhat confused, owing to his imperfect 
apprehension of the true nature of the phenomenon 
which he observed, but, without doubt, he observed a 
true magnetic effect on the radiated light—namely, this 
NO. 1547, VOL. 60] 
broadening of the spectral lines—but he did not convince 
the scientific world that he had made any new discovery, 
and so the matter fell into neglect until it was revived 
again in 1897 by the now celebrated work of Dr. P. 
Zeeman. 
Work of Zeeman, Lorentz, and Larmor. 
The credit which attaches to Dr. Zeeman’s work is 
that he not only, after prolonged effort, succeeded in 
obtaining this new magnetic effect, but he also convinced 
the world that the effect was a true one, arising from the 
action of the magnetic field on the source of light. That 
Dr. Zeeman was able to do this was due, perhaps, as much 
to the present advanced state of our theoretical know- 
ledge of this subject as to his own skill and perseverance 
as an observer; and this is a striking example of the 
great assistance which well-founded theory affords to 
experimental investigation. The theory connects the 
facts already known in reasonable and harmonious 
sequence, predicts new results, and points out the 
channels through which they must be sought. Without 
such scientific theory, this general systematic advance 
would be impossible, and new results would be stumbled 
on only by accident. 
To see how this applies to our case, we revert to the 
fact determined by Dr. Zeeman—namely, that when the 
source of light is placed in a strong magnetic field the 
spectral lines become broadened (see Fig. 1). As soon as 
Fic. 1.—Shows the broadening of the spectral lines by the magnetic 
field. The upper row shows the lines when the magnet is not 
excited. The lower row shows the same lines when the magnet Is 
excited. (Reproduced from a photograph, natural size.) 
this was announced, Prof. Lorentz, and subsequently Dr. 
Larmor, examined the question from the theoretical point 
of view. They analysed the subject mathematically, and 
came to the conclusion that each spectral line should be 
not merely broadened, but should be actually split up into 
three—that is, each line should become three lines, or, as 
we shall say in future, a triplet. They also arrived at the 
further most important and interesting conclusion, viz., 
that the constituent lines of this triplet must be each 
plane polarised—the central line of the triplet being 
polarised in one plane, while the side lines are polarised 
in a perpendicular plane. In fact, the vibrations of the 
light forming the central line are parallel to the lines of 
magnetic force, while the vibrations in the side lines are 
perpendicular to the lines of force. This prediction of 
tripling and polarisation from theoretical considerations 
may be regarded as the key to the subsequent advance 
that has been made in the investigation of this region of 
physics. In order to understand it, let us place ourselves 
in Dr. Zeeman’s position when he found that the spectral 
lines became broadened by the magnetic field, and let us 
be informed that this broadening is in all probability a 
tripling of the lines accompanied by plane polarisation. 
The question now is, ‘‘ How are we to determine if this 
is the case?” 
