178 
IVA TCE: 
[JUNE 22, 1899 
therefore, the experimental investigation which yields 
more than the simple theory expected is not to be taken 
as in any way discordant with that theory, but on the 
contrary to be in harmony with it. 
Theoretical Considerations. 
In order that you may form some idea as to what it is 
‘that the theory supposes to be in operation in the pro- 
duction of these phenomena, I have had this elliptic 
frame constructed [model shown], which I ask you for 
the present to consider as the orbit described by one of 
those elements of matter which by their motions set up 
waves in the ether, and thereby emit what we call light. 
This white ball, which slides on the elliptic frame, is 
supposed to represent the element of matter. It is some- 
times called an ion, which name is used to imply that the 
element of matter carries an electric charge inherently 
associated with it. 
Now, under ordinary circumstances this ion revolving 
in its orbit with very great rapidity will continue to do 
so peacefully unless external forces come into play to 
disturb it. When external forces come into action, the 
orbit ceases in general to be the same as before. The 
orbit becomes perturbed, and the external forces are 
termed perturbing forces. But you now ask what is the 
character of the forces introduced by the magnetic field 
when the ion is moving through it. In answering this, 
we are to remember that the ion is supposed to be an 
element of matter charged with an electric charge—or, 
if you like, an electric charge possessing inertia. Now, 
if a charged body moves through a magnetic field, it is 
an experimental fact that it experiences a force arising 
from the action of the magnetic field on the moving 
electric charge. The direction of this force is at right 
angles both to the direction of motion of the charged 
body and to the direction of the magnetic force in the 
field. The effect of this force in our case is to cause the 
elliptic orbits of the ions to rotate round the lines of 
magnetic force; or to cause them to have a pre- 
-cessional motion [illustrated by model] instead of staying 
fixed in space, just as the perturbing forces of the planets 
in the solar system cause the earth’s orbit to have a 
precessional motion. The angular velocity of this pre- 
-cessional motion is proportional to the strength of the 
magnetic field, and depends also, as you would expect, on 
the electric charge and the inertia associated with the ion. 
This precessional motion of the orbit, combined with 
the motion of the ion around the orbit, gives the whole 
motion of the ion in space, and the result of this com- 
bined movenient, of these two superposed frequencies— 
viz. the frequency of revolution of the ion in its orbit, 
and the frequency of rotation of the orbit around the 
lines of force—is that, in the case of the light radiated 
across the lines of force, each period becomes associated 
with two new periods, or, in other words, each spectral 
fine becomes a triplet. A partial analogue to this, 
which may to some extent help you to understand the 
introduction of the two new periods, occurs in the case of 
sound, although the two phenomena at basis are quite 
different. The analogue (or quasi-analogue) is this. 
When two notes of given pitch, that is, of given frequency 
of vibration, are sounded together, their superposition 
produces two other notes of frequencies which are 
respectively the sum and the difference of the frequencies 
of the two given notes. These are known as the sum- 
mation and the difference tones of the two given 
notes. Corresponding to these are the two side lines of 
the magnetic triplet. The frequency of the vibration in 
one of these lines is the sum, and the frequency of the 
other is the difference, of the two frequencies mentioned 
before—namely, the frequency of the revolution of the 
ion around its orbit and the frequency of the precessional 
revolution of the orbit round the lines of force. The 
vibration, and this frequency disappears completely 
when the light is viewed along the lines of force—that 
is, through axial holes pierced in the pole-pieces. In 
this direction, too, a further peculiarity arises, for not 
only does the triplet drop its central member and become 
a doublet, but each member of this doublet is not plane 
polarised, as the members of the triplet are. They are 
each, on the contrary, circularly polarised—that is, the 
vibration is circular instead of being rectilinear. 
This all follows as the expectation of the simple 
theory which supposes that the ions are free to describe 
their elliptic orbits undisturbed by any forces other 
than the magnetic field. But it is only to be expected 
that other perturbing forces must come into play in the 
assemblage of ions which build up incandescent matter 
of the source of light. We know, for example, that the 
other members of the solar system perturb the earth’s 
motion, so that it deviates from the simple elliptic motion 
predicted by the simple theory which did not take these 
perturbing forces into account. Hence, if any such 
perturbing forces exist, and we should be surprised if 
they did not exist, the tripling pure and simple of the 
spectral lines will be departed from, and other types will 
arise. From the character of these new types we may 
infer the nature of the perturbations which give rise to 
them, and hence by the study of these types we obtain 
a view of what is going on in matter when it is emitting 
light, which we should not possess if such perturbations 
did not occur. These deviations from pure tripling are 
consequently of more importance almost, in regard to 
our future progress, than the discovery of the tripling 
itself. To give you some idea of the influence of such 
perturbations in modifying the triplet form, I may 
mention that it follows, from simple theoretical con- 
siderations, that if the perturbing forces cause the orbit 
to revolve in its own plane, or cause it to change its 
ellipticity periodically, then each line of the triplet pro- 
duced by the magnetic field will be doubled and a sextet 
will result, and other oscillations of the orbit will give 
rise to other modifications of the normal triplet type. It 
is not quite easy to see at once, however, what the 
perturbing forces are exactly, for we do not know the 
way in which the ions are associated in matter ; but if we 
regard an ion as a charged element of matter describing 
an orbit, it will be analogous to a closed circuit, or to a 
magnetic shell, and will be urged to set in some definite 
way in the magnetic field. In coming into this position, 
it may oscillate about the position of equilibrium, and 
thus introduce an oscillation into the precessional 
motion of the orbit, which may have the effect of 
doubling or tripling the constituents of the pure 
precessional triplet. 
Now, experimental investigation shows us that all the 
spectral lines do not become triplets when viewed across 
the lines of force in a magnetic field, for some lines show 
as quartets, or sextets, or octets, orin general as complex 
triplets derived from the normal triplet by replacing 
each component by a doublet or a triplet. We conclude, 
therefore, that the ions which give rise to these complex 
forms are not perfectly free in their motions through the 
magnetic field, but are constrained in some way by 
association with each other in groups, or otherwise, while 
they move in the magnetic field. 
Law of the Magnetic Effect. 
And now we come to a very important point in this 
inquiry. According to the simple theory, every spectral 
line, when viewed across the lines of force, should 
become a triplet in the magnetic field, and the differ- 
ence of the vibration frequency between the side lines 
of the triplet should be the same for all the spectral 
lines of a given substance. In other words, the pre- 
cessional frequency should be the same for all the 
centre line of the triplet has the frequency of the original | lonic orbits, or the difference of wave-length 6A between 
NO. 1547, VOL. 60| 
