350 
NATURE 
[Aucust 17, 1899 
prism, and two others in which the wave is stopped. The 
prism can be turned from one position to another by properly 
placing it and then turning it round the direction of the ray. It 
is found that if the prism be thus turned from a position in 
which the light is freely transmitted we come after turning it 
through 90° to a position in which the light is stopped, and that 
if we go on turning through another angle of 90° a position is 
reached in which the light is again freely transmitted, and so on, 
the light being alternately stopped and transmitted by the second 
prisms in successive positions go” apart. ‘ ' 
The mode of passage of the wave by the Nicols when their 
planes are parallel, and its stoppage when the planes are 
crossed, are illustrated by this diagram (Fig. 3) of a vibrating 
cord and two slits. When the slits are parallel, the vibration 
Fic. 3 
which is passed by one is passed by the other; when they are 
crossed, a vibration passed by one is stopped by the other. 
Two planes of symmetry of the prisms parallel to the ray, 
and called their principal planes, are parallel to one another 
when the light passes through both, and are perpendicular to 
one another when the light passed by the first is stopped by the 
second. We shall call the first prism the polarising prism, or 
the folarzser, from its effect in producing plane polarised light ; 
the other, the azalyser. The stoppage of the light in the two 
positions 180° apart of the second prism and its passage in the 
two intermediate positions show that the light passed by the 
first prism is plane polarised. 
Now a beam of plane polarised light is passed through the 
perforated pole-pieces of this large electro-magnet (Fig. 4), so 
that the beam travels between the pole faces along the direction 
which the lines of force there would have if the magnet were 
excited by a current. The arrangement of the apparatus is as 
shown in the diagram. The light is polarised by the prism P, 
passes through the magnetic field, and then through the analys- 
ing prism A, to the screen. As you see, when the second prism 
is turned round the ray the light on the screen alternately 
shines out and is extinguished, and you can see also that the 
angle between the positions of free passage and extinction 
is 90°. 
I now place in the path of the beam this bar of a very re- 
markable kind of glass, some of the properties of which were 
nvestigated by Faraday. It is a very dense kind of lead glass, 
NO. 1555, VOL. 60] 
which may be described as a silicated borate of lead ; that is, it 
contains silica, boric acid and lead oxide. The beam is not 
disturbed although the light passes through the glass from end 
to end. I now adjust the analysing prism to very nearly com- 
plete extinction, and then excite the magnet. If the room is 
sufficiently darkened I think all will see that when the magnet 
is excited there is a very perceptible brightening of the dim 
patch of light on the screen, and that this brightening dis- 
appears when the current is removed from the magnet. This 
is Faraday’s discovery. 
How are we to describe this result? What effect has been 
produced by the magnetic field? It is clear that the direction 
of vibration of the light emerging from the specimen of heavy 
glass has been changed relatively to the prism so that the 
light now readily passes. It is found, moreover, that the 
amount of turning of the direction of vibration round the ray 
is proportional to the length of the specimen, so that the 
directions of vibration at different points along the wave within 
the specimen lie on a helically twisted surface, and may be 
regarded as represented by the straight rods in the model before 
you on the table (Fig. 5). 
It is also found that the amount of the turning depends on 
the intensity of the magnetic field—is, in fact, simply propor- 
tional to that intensity. Hence the turning is proportional to 
the mean intensity of the field, and to the length of the path 
in the medium, that is, to the products of these two quantities. 
It also depends on the nature of the medium. The angle of 
turning produced bya field of known intensity when the ray 
passes through bisulphide of carbon has been very carefully 
measured by Lord Rayleigh, whose results are of great value 
for other magnetic work. 
The law of proportionality of the amount of turning of the 
plane of polarisation to the intensity of the magnetic field in 
Fic. 5. 
the space in which the substance is placed is not, however, to 
be regarded as established for strongly magnetic substances, 
such as iron, nickel or cobalt, The matter has not yet been 
completely worked out, but the turning in such cases seems to 
be more nearly proportional to the intensity of magnetisation, 
a different quantity from the intensity of the magnetic field pro- 
ducing the magnetisation. If this law be found correct the 
angle of turning will be proportional to the product of the 
intensity of magnetisation and to the length of the path ; and 
the angle observed divided by this product will give another 
constant, which has been called Kundt’s constant. 
The rotation of the plane of polarisation in strongly mag- 
netised substances was investigated by Kundt, the very eminent 
head of the Physical Laboratory of the University of Berlin, 
who died only a year or two ago. Kundt is remembered for 
many beautiful methods which he introduced into quantitative 
physical work ; but no work he did was more remarkable than 
that which he performed in magneto-optic rotation when he suc- 
ceeded in passing a beam of plane polarised light through plates 
of iron, nickel and cobalt. Such substances, though apparently 
opaque to light, are not really so when obtained in plates of 
sufficient thinness. In sufficiently thin films all metals, so far 
as I know, are transparent, not merely to Réntgen rays, but to 
ordinary light. Kundt conceived the idea of forming such 
films of the strongly magnetic metals, so as to investigate their 
properties as regards magneto-optic rotation. He succeeded in 
obtaining them by electroplating platinised glass with such thin 
ps 
