of Edinburgh^ Session 1863 - 64 . 227 
This argument, which 'prima facie seems so ingenious and plausible, 
is, nevertheless, in my opinion, void of foundation, because it rests 
on premises which are empirically and philosophically inadmissible. 
In the first place, empirically^ because the hypothesis postulates, 
that a ray of light is propagated, both within and l^eyond the polar- 
ising medium, in the form of a spiral ; whereas Monsieur Fresnel, 
another distinguished French philosopher, has clearly proved, both 
experimentally and mathematically, that the plane of polarisation 
assumes this twisted form in virtue ©f the mutual interference of 
two pencils of light, into which the original beam has resolved 
itself, and the particles of which, instead of vibrating as formerly 
in straight lines, are henceforth made to vibrate in the direction 
of a curve, and across two opposite regions of the influencing 
molecule. In the second place, philosophically, because the hypo- 
thesis places us between the horns of the following dilemma : 
either to make the optical change dependant upon a purely me- 
chanical cause, namely, upon the assumption of a pre-existing 
spiral form of arrangement, in consequence of which a beam of 
light is nolens volens compelled to travel along the solid walls of a 
winding tunnel, or to make the optical change dependant upon a 
purely dynamical cause, namely, upon the assumption of a pre- 
existing unsymmetrical disposition of the atoms within the limits 
of each individual molecule ; in consequence of which the mole- 
cule is supposed all at once to acquire the power of turning the 
luminous particles from their rectilinear course. 
Now, it is a well known axiom of cosmical economy, that, in try- 
ing to interpret facts, we ought not to multiply causes unneces- 
sarily, and that one and the same class of phenomena ought, if 
possible, to be referred to one cause only ; but when, as in the case 
before us, these causes are so essentially distinct, both in their 
form and in their mode of action, the arguments employed by 
Monsieur Pasteur, unsupported as they are by either experiments 
or calculations, appear to me no longer tenable, and it behoves us, 
therefore, to search for a more consistent and comprehensive ex- 
planation. In reviewing the new order of phenomena above 
alluded to, and which embraces the following principal physical 
properties of matter, — viz. specific gravity, specific volume, allo- 
tropism and polymorphism, fusing and boiling points, crystalline 
