Aucust 17, 1899] 
NATURE 
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the ethnological specimens brought back from the Pacific | work on the relation of magnetism to light has been founded. I 
islands by the Wilkes Exploring Expedition. 
Bowers has notified the Smithsonian Institution that the naval 
and civil attachés of the vessel will be given special instructions 
to be on the look-out for desirable ethnological material. 
There is every reason to believe that this expedition will 
yield valuable scientific results, and will be creditable to the | 
United States. It is the most important marine expedition on | 
which the Fish Commission has embarked, and one of the most 
promising scientific enterprises in which the U.S. Government | 
has ever engaged. It is a matter for congratulation that, in the 
activity in exploration of the seas now being exhibited by various 
Governments, the United States will participate under such | 
Commissioner | am permitted by the kindness of the authorities of this Insti- 
tution to exhibit here the very apparatus which Faraday himself 
employed, though for the various experiments I have to make 
it is necessary to actually use another set Of instruments. [4/- 
paratus shown.| Before repeating Faraday’s experiment, let 
me describe shortly what I propose to do, and the effect to be 
observed. 
A beam of plane polarised light is produced by passing white 
light from this electric lamp through a Nicol’s prism. To under- 
stand the nature of plane polarised light, look for a moment at 
this other diagram (Fig. 1). It represents a series of particles 
displaced in a certain regular manner to different distances from 
favourable auspices and be represented by a man of science of | the mean or equilibrium positions they originally had along a 
such wide experience in deep-sea investigation as Prof. Agassiz. 
MAGNETO-OPTIC ROTATION AND ITS 
EXPLANATION BY A GYROSTATIC SYSTEM. 
“THE action of magnetism on the propagation of light in 
a transparent medium has been rightly regarded as one of 
the most beautiful of Faraday’s great scientific discoveries. 
Like most important discoveries it was no result of accidental 
observation, but was the outcome of long and patient inquiry. 
Guided by a conviction that (to quote his own words) ‘‘ the 
straight line. They are moving in the directions shown by the 
arrows and with velocities depending on their positions, as in- 
dicated by the lengths of the arrows. Suppose a certain interval 
of time to elapse. The particles will have moved in that time 
to the positions shown in this other diagram (Fig. 2) on the 
same sheet. It will be seen that the velocities as well as the 
positions of the particles have altered ; but that the configuration 
is the same as would be given by the former diagram moved 
through a certain distance to the left. 
Thus an observer looking at the particles and regarding their 
configuration would see that configuration apparently move to 
the left ; and this, it is very carefully to be noted, is a result of 
various forms under which the forces of matter are made mani- | 
fest have one common origin,” he made many attempts to | 
discover a relation between light and electricity, but for very 
long with negative results. 
persuasion that his view was correct, and that some such re- | 
lation must exist, he was undiscouraged, and only proceeded to 
search for it more strictly and carefully than ever. At last, as 
he himself says, he ‘* succeeded in magnetising and electrifying 
a ray of light, and in illuminating a magnetic line of force.” 
Faraday pictured the space round a magnet as permeated by | 
what he called lines of force ; these he regarded as no mere | 
mathematical abstractions, but as having a real physical existence | 
represented by a change of state of the medium brought about | 
by the introduction of the magnet. That there is such a 
medium surrounding a magnet we take for granted. The lines 
of force are shown by the directions which the small elongated | 
| sulting from that vibration. 
the transverse motions of the individual particles. In another 
interval of time equal to the former the arrangement of particles 
will appear to have moved a further distance of the same amount 
Still, however, retaining a strong | towards the left. 
This transverse motion of the particles, thus shown displaced 
from their equilibrium positions, represents the vibration of the 
medium which is the vehicle of light, and the right to left 
motion of the configuration of particles is the wave motion re- 
I do not say that the medium is 
thus made up of discrete particles, or that the different portions 
of it vibrate in this manner, but there is undoubtedly a directed 
quantity transverse to the direction in which the wave is travel- 
ling, the value of which at different points may be represented 
by the displacements of the particles, and which varies in the 
same manner, and results, as here shown, in the propagation of 
a wave of the quantity concerned. 
Fic. 
pieces of iron we have in iron filings take when sprinkled on a 
smooth horizontal surface surrounding a horizontal bar magnet, 
as in the experiment I here make. [A xferviment to show field 
of bar magnet by tron filings.) 
The arrangement of these lines of force depends upon the 
nature of the magnet producing them. If the magnet be of 
horse-shoe shape, the lines are crowded into the space between 
the poles ; and if the pole faces be close together and have their 
opposed surfaces flat and parallel the lines of force pass straight 
across from one surface to the other in the manner shown in the 
diagram before you. [Diagram of field between flat pole faces. | 
The physical existence of these lines of force was demonstrated 
for a number of different media by the discovery of Faraday to 
which I have already referred, and on which almost all the later 
a delivered at the Royal Institution by Prof. Andrew Gray, 
NO. 1555, VOL. 60] 
In fact, we have here a representation of a wave of plane 
polarised light. The directions of vibration are right lines 
parallel at all points along the wave. Ordinary light consists 
of vibrations the directions of which are not parallel if recti- 
linear, and each vibration is therefore capable of being resolved 
into two in directions at right angles to one another. The 
Nicol’s prism, in fact, splits a wave of ordinary unpolarised 
light into two waves, one in which the vibrations are in one 
plane containing the direction in which the light is travelling, 
the other in a plane containing the same direction, but at right 
angles to the former. One of these waves is stopped by the 
film of Canada balsam in the prism and thrown out of its 
course, while the other wave is allowed to pass on undisturbed. 
If the wave thus allowed to pass by one Nicol’s prism be 
received by another it is found that there are two positions of 
the latter in which the wave passes freely through the second 
