85 
of Edinburgh , Session 187 5-7 6. 
It will be seen that the relation between appended weights and 
thereby increased resistances is not that of simple proportion. In 
this respect silver wires appear again to differ from copper wires. 
Some experiments made by Messrs Meik and Murray * having 
shown that the changes of resistance of copper wires, when 
stretched by weights, are directly proportional to the weights. 
I am deeply indebted to Professor Wiedemann of Leipzig, in 
whose laboratory these experiments were performed, for the excel- 
lent apparatus which he kindly placed at my disposal, and for the 
advice and assistance with which he favoured me. 
2. On the Defoliation of the Coniferse. By Dr Stark. 
3. On Diamagnetic Rotation. By George Forbes, Esq. 
M.A., F.R.A.S. 
Faraday’s discovery of the magnetic rotatory polarisation of 
light may be expressed in the following manner : — Let two electro- 
magnets, in the form of iron tubes, surrounded by helices of wire, be 
placed end to end, so that in the spaoe between them the lines of 
force are very intense. Let a rod of dense glass be placed in this 
space, so that a ray of light may pass through the two tubes and 
the rod of glass. Let such a ray on entrance be plane-polarised, 
so that the direction of vibration is in a vertical direction. If the 
electro-magnet be now magnetised, the emergent ray will be 
polarised, so that its vibrations are inclined to the vertical at a 
small angle. The direction in which the line of vibration has been 
rotated is the same as the direction of the positive current in the 
helices. 
The same effect might be produced without the aid of mag- 
netism if the rod were rotated round the axis of the ray of light 
with great velocity. The rotation of the plane of polarisation 
* Proc. Roy. Soc. Edin., Session 1869-70, p. 3. 
