1876.] 
The British Association. 
5 6 * 
plate of metal, and were viewed by light polarised perpendicularly to the 
plane of incidence, it was known that, as the angle of incidence was increased, 
the rings which were at first dark-centred, disappeared in passing the 
polarising angle of the glass, and then reappeared white-centred, in which 
state they remained up to a growing incidence, when they could no longer be 
followed. At a high incidence the first dark ring was much the most 
conspicuous. To follow the rings beyond the limit of total internal reflection, 
a prism must be employed. When the rings formed between glass and glass 
were viewed in this way, as the angle of incidence was increased the rings one 
by one opened out, uniting with bands of the same respective orders which 
were seen beneath the limit of total internal reflection; the limit or boundary 
between total and partial reflection passed down beneath the point of contact, 
and the central dark spot was left isolated in a bright field. Now, when the 
rings were formed between a prism with a slightly convex base and a plate of 
silver, and the angle of incidence was increased so as to pass the critical 
angle, if common light be used in lieu of a simple spot, we had a ring which 
became more conspicuous at a certain angle of incidence well beyond the 
critical angle, after which it rapidly contracted and passed into a spot. To 
study the phenomenon in its purity it is necessary to employ polarised light, 
or, which is more convenient, analyse the reflected light by means of a prism. 
This phenomenon was discovered by Professor Stokes many years ago, but he 
has only just completed the necessary investigations. 
Dr. Ker, to whose valuable discovery of the double refraCting property 
produced in a dieleCtric by eleCtric induction we referred in a recent number 
of this journal, described “An Experiment proving Rotation of the Plane of 
Polarisation of Light Reflected from a Magnetic Pole.” If plane polarised 
light be allowed to fall on the polished extremity of the soft iron core of an 
eleCtro-magnet, and a Nicol’s prism be fixed in such a position as to extinguish 
the reflected light, the current being not yet sent through the coil of the 
magnet, on sending the current, the light is restored. In order to increase the 
magnetisation of the reflecting surface, or that portion of it which is utilised, 
the wedge-shaped termination of a mass of iron is held close over it. Sir W. 
Thomson remarked that this experiment proved iron to possess, in an enor- 
mously higher degree, the property discovered by Faraday for heavy glass. 
In a paper “ On the Protection of Buildings from Lightning,” Professor 
Clerk Maxwell proposed to take advantage of the faCtthat an eleCtric discharge 
cannot occur between two bodies unless their difference of potential was 
sufficiently great compared with the distances between them, but it was shown 
by experiment that if every part of the surface surrounding a certain region 
is at the same potential, every point within that region must be at 
the same potential, provided no charged body is within the region. If a 
powder mill were coated on the roof, walls, and floor with sheet copper, if all 
conductors, such as water-pipes, entering the building were connected with the 
coating, no electrical effeCt could be produced in the interior : but in practice, 
even this was superfluous in this climate. For ordinary buildings it would 
suffice to have a copper wire carried round the foundation of the house, up 
each of the corners and gables, and along the ridges. The copper wires 
might be built into the walls to prevent theft. In the case of a powder-mill, 
it might be advisable to make the network closer by carrying wires over the 
walls. It was advisable not to ereCt a tall condaCtor with a sharp point in 
order to relieve the thunder clouds of their charge. 
A paper “ On the Influence of the Residual Gas on the Movement of the 
Radiometer ” was read by Mr. Crookes. His recent experiments show that the 
movement of this instrument is not due to a direCt repulsion exerted by light 
on the vanes, but to a mutual action called out between these vanes and the 
very attenuated gas remaining in the instrument. It is well known that, with 
a moderately good vacuum, the motion becomes more rapid as the exhaustion 
proceeds ; but Mr. Crookes has recently succeeded in producing such a com- 
plete exhaustion that he not only reaches the point of maximum effedl, but 
goes far beyond it, so far that the effect nearly ceases. He measures the 
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