of Edinburgh, Session 1875-76. 
91 
piece of Faraday’s heavy glass. But in this case the glass was 
inserted in a hole in a cork, to which also the mirror was attached. 
On examining the cork it was found to be magnetic. This 
accounted for all the phenomena observed. 
But in the previous session this would not account for the 
phenomena. There was no cork employed ; and experiments 
made to test this by altering the position of the magnet showed 
that there was no magnetism in the arrangement. 
It was the doubt that hung over the last winter’s experiments 
that made me wish to delay publishing any results until I should 
have finally settled the matter. I have been unable to do so 
hitherto, and offer the original experiments in the meantime. 
Note on the preceding paper. 
[The first statement is that a rotation of the plane of polarised 
light might be produced by rotating a transparent body about the 
ray as an axis. 
It is improbable that no such effect would be produced, but that 
the question is by no means a simple one may be seen by looking at 
Sir W. Thomson’s paper on this subject (Proc. R. S. Bond., 1856). 
I have also tried a great many hypotheses besides those which I 
have published, and have been astonished at the way in which 
conditions likely to produce rotation are exactly neutralised by 
others not seen at first. 
There can be no doubt, however, that a rotation of some kind is 
going on in a diamagnetic medium under magnetic force, and this 
may be of the molecules of the glass of the ether or what not, 
and this probably goes on in all media whether transparent or not. 
This rotation, as Prof. G-eorge Forbes says, stops as soon as the 
magnetic force is removed. He supposes that it is stopped by 
friction, and therefore, that energy is being dissipated at all 
times as long as magnetic force acts on a medium. 
But we know that a magnet will retain its magnetism for a long 
time, and it has never been shown that a magnet must necessarily 
lose its magnetism. Hence we must admit that the molecular 
rotation is not accompanied with friction, but that it is set up by 
