TRANSACTIONS OF THE SECTIONS. 23 
On the Theory of Magnetic Induction in Crystalline Substances. 
~ By Prof. W. THomson. 
Pliicker’s admirable discovery of the directive action experienced by crystals in a 
magnetic field, renders it an object of the highest interest to establish a theory of 
magnetic induction in crystalline substances. A theory founded on Poisson’s ori- 
ginal suggestion regarding the possible magnetic structure of crystalline matter, 
which has exclusive reference to the hypothesis, now universally rejected, of ‘‘ mag- 
netic elements,” each containing “ northern and southern magnetic fluids” in equal 
quantities, could not be received in the present state of science. ‘The author of this 
communication stated two principles, involving no physical hypothesis regarding the 
ultimate nature of magnetization, which he considered to be a sufficient foundation for 
a complete mathematical theory of magnetic induction. One of these, which he calls 
the superposition of magnetic inductions, cannot be considered as fully established by 
experiment ; but it is probably true, or approximately true, in a great variety of actual 
cases, especially those in which the capacity for magnetic induction is very feeble. 
On the other hand, it is not probable that it is even approximately applicable to soft 
iron in a state of intense magnetization (such as may be produced by electro-mag- 
netic means), since it is hardly to be conceived that iron in such a state could be as 
open to additional magnetization from another magnet as non-magnetized soft iron. 
The theory indicates with clearness and precision ‘how any deviations from this 
principle, which experiment may point out, are to be taken account of, The author 
proceeded to indicate some of the conclusions which may be drawn by mathematical 
reasoning, from the two principles which he had stated, and gave the steps of a demon- 
stration of the existence of three axes at right angles to one another in every crystal- 
line substance, possessing certain symmetrical properties with reference to inductive 
magnetization, in virtue of which they may be called ‘‘ principal axes of magnetic 
induction.” 
In conclusion the author remarked, that, although on first reading a paper ‘‘on 
the Magneto-optic Properties of Crystals ”’ by Messrs. John Tyndall and Hermann 
Knoblauch, recently published in the Philosophical Magazine (July), he considered 
that there would be some difficulty in reconciling the views of these writers with his 
theory; yet the opportunity the British Association had afforded him of discussing 
personally with Mr. Tyndall some of the points of difference, gave him reason for 
hoping that a complete agreement would ultimately be established. 
On the Magneto-Optical Properties of Crystals. By Joux TynDALt. 
AstTRoNoMy, Mrreors, WAVEs. 
On a Sidereal Clock for showing the Arc of Right Ascension directly. 
By Prof. CHEvAtvier, F.R.S. 
_ On the alleged evidence for a Physical Connexion between Stars forming 
Binary or Multiple Groups, deduced from the Doctrine of Chances. By 
James D. Fores, F.R.S., Corr. Member of the Institute of France, §e. 
an opinion has long obtained amongst astronomers that the great number of 
cases which occur in the heavens of two or more stars being apparently very close 
to one another (constituting what have been called double, triple or multiple stars), 
constitutes of itself an argument for a more than apparent connexion between the 
members of those groups. 
It is evident that two stars may constitute an apparently double star without 
any real proximity between them, merely because a line passing through the eye of 
the spectator and the nearer star may, if prolonged into space (no matter how far), 
pass somewhere near a second star, whose position would therefore seem almost to 
coincide with the first, although the distance which separates them might be indefi- 
nitely great. Such stars are sometimes said to be ‘‘ optically”’ double. On the other 
