Aucust 17, 1899] 
NATURE 
Ca) 
j01 
stratums of these metals that light passed through them in 
sufficient quantity for observation. The rotation produced by 
the glass and the exceedingly thin film of platinum was deter- 
mined once for all and allowed for. Kundt obtained the 
remarkable result that the magnetic rotatory power in iron is so 
great that light transmitted through a thickness of one centi- 
metre of iron magnetised to saturation is turned through an 
angle of over 200,000°, that is, that light passing through a 
thickness of an inch of iron magnetised to saturation would 
have its plane of polarisation turned completely round more 
than a thousand times; in other words, one complete turn 
would be given by a film less than j9/55 of an inch in thickness. 
A scarcely smaller result has been found by Du Bois for cobalt, 
and a maximum rotation of rather less than half as much by the 
same experimenter for nickel. 
The direction of turning in all the cases which have so far 
been specified—that is, Faraday’s glass, bisulphide of carbon, 
iron, nickel and cobalt—is the same as that in which a current 
of electricity would have to flow round the spires of a coil of 
wire surrounding the specimen so as to produce the magnetic 
field. This we call the fovz¢zve direction. There are, how- 
ever, many substances in which the turning produced by the 
magnetic field is in the contrary or negative direction; for 
example, ferrous and ferric salts of iron, chromate and 
bichromate of potassium, and in fact most compound substances 
which are feebly magnetic. 
Faraday established by his experiments the fact that sub- 
stances fall into two distinct classes as tested by their behaviour 
under the influence of magnetic force. For example, au 
elongated specimen of iron, nickel or cobalt, if freely suspended 
horizontally between the poles of our electro-magnet, would 
set itself with its length along the lines of force. On the 
other hand, a similar specimen of heavy glass, or a tube filled 
with bisulphide of carbon, would, if similarly suspended, set 
itself across the lines of force. The former substances were 
therefore called by Faraday paramagnetic, the latter dia- 
magnetic. 
It might be supposed that diamagnetics would show a turning 
effect opposed to that found in paramagnetics, but this is not the 
case. As we have seen, bisulphide of carbon and heavy glass, 
which are diamagnetics, show a turning in the same direction as 
that produced in iron—as indeed do most solid, fluid and gaseous 
diamagnetics. Feebly paramagnetic compound substances, on 
the other hand, produce negative rotation. 
A theory of diamagnetism has been put forward in which the 
phenomena are explained by supposing that all substances are 
paramagnetic in reality, but that so-called diamagnetic bodies 
are less so than the air in which they are immersed when 
experimented on. Thus the diamagnetic quality is one of the 
substance relatively to air, in the same kind of way as the 
apparent levity of a balloon is due to the fact that its total 
NO. 1555, VOL. 60] 
weight has a positive value, but is less than that of the air dis- 
placed by the balloon andappendages. Lord Kelvin’s dynamical 
explanation of magneto-optic rotation does not bear out this 
view of the matter. 
Before passing to the dynamical explanation, however, I must 
very shortly call attention to some remarkable discoveries in this 
subject made by Dr. John Kerr, of Glasgow. I have here an 
electro-magnet arranged as in the diagram before you (Fig. 6). 
The light from the lamp is first plane polarised by the Nicol P, 
then it is thrown on the piece of silvered glass G, and part of it 
is thereby reflected through this perforated pole-piece so as to 
fall normally on the polished point of the other pole-piece. 
Reflection thus takes place at perpendicular incidence, and the 
reflected light is received by this second Nicol. When the 
magnet is unexcited the second Nicol is arranged so as to quench 
the reflected light. The magnet is then excited, and it is found 
that the light is faintly restored, showing that an effect on the 
polarisation of the light has been produced by the magnetisation. 
It is to be noticed here that the incident and reflected light is in 
the direction of magnetisation. We shall not pause to make 
this experiment. It was arranged this morning and successfully 
carried out ; but the effect is slight, and might not be noticeable 
without precautions, which we have hardly time to make, to 
exclude all extraneous light from the screen. 
It would perhaps be incorrect to say that the plane of polar- 
isation has been rotated in this case, as it has been asserted by 
Righi that the light after reflection is no longer plane polarised, 
but that there are two components of vibration at right angles 
to one another, so related that the resultant vibration is not 
There is therefore no position im 
which the analysing prism can be placed so as to extinguish the 
rectilinear but elliptical. 
reflected light. The transverse component necessary to give 
the elliptic vibration is, however, in this case, if it exists, very 
small, and very nearly complete extinction of the beam can be 
obtained by turning the analysing prism round so as to stop the 
other component vibration. The angle through which the 
prism must be turned to effect this is the amount of the apparent 
rotation. The direction of rotation is reversed by reversing the 
magnetism of the reflecting pole. Dr. Kerr found that the 
direction is always that in which the current flows in the coils, 
producing the magnetisation of the pole. 
Dr. Kerr also made experiments with light obliquely incident 
on a pole-face, with the arrangement of apparatus shown in this. 
other diagram (Fig. 7). He found that the previously plane 
polarised light was by the reflection rendered slightly elliptically 
polarised. A slight turning of the analysing Nicol was neces- 
sary to place it so as to stop the vibration corresponding to the 
long axis of the ellipse and so secure imperfect extinction. 
These effects are, like those of normal incidence, very small, 
and they can hardly be shown to an audience. 
(Zo be continued. ) 
THE annual Report of the Department of Science and Art 
furnishes much information on the progress made in 
elementary scientific instruction year by year; and the following 
facts, derived from the Report just published, shows the vast 
extent of the Department’s operations during 1898. The 
number of students under instruction in schools eligible for the 
1 Forty-sixth Report of the Department of Science and Art of the Com- 
mittee of Council on Education, with appendices. Pp. 320. (H.M- 
Stationery Office, 1899.) 
