506 Prof. Challis on Theoretical Physics. 
be a reality, is another question. It does not admit of @ priori 
proof or disproof, but may be disproved by a single contradictory 
fact, or may receive accumulative evidence by the agreement of 
its mathematical results with many facts. Now I venture to 
assert respecting this particular hypothesis, that the mathema- 
tical evidence of its truth and of the reality of a fluid ether 
is so varied and comprehensive, that it may be pronounced to 
be all but conclusive. My reasons for this assertion are the fol- 
lowing :—When a mathematical inquiry is made into the laws of 
the motion and pressure of a fluid constituted as above supposed, 
certain results are obtained by the formation and solution of par- 
tial differential equations which correspond to various pheno- 
mena of light. The difference of the intensities of different rays, 
the variation of intensity with the distance from a centre, and 
the law of the variation, the coexistence at the same instant of 
different portions of light in the same portions of space, the 
interference and non-interference of different rays, the composite 
character of light, its colour, results of compounding colours, 
and lastly the polarization of light, are all phenomena which 
have their exact analogues in the motions, as mathematically 
deduced, of a fluid medium whose pressure varies as its density. 
When the number, variety, and speciality of these analogies are 
considered, it seems difficult to resist the conclusion that proper- 
ties of the fluid ether explain phenomena of light, and that the 
phenomena reciprocally give evidence of the reality of the ether. 
Some of the properties—for instance, that of transverse vibration, 
which accounts for polarization—have been deduced by mathema- 
tical reasoning for which I am responsible. I have, however, 
given to mathematicians the fullest opportunity of discussing 
these parts of the general argument ; and when, as I hope to be 
able to do, I go through a revision of the propositions, further 
opportunity will be given. ‘The proof of the reality of the 
eetherial medium, drawn from the explanations which the hypo- 
thesis of such a medium gives of phenomena of light, is an essen- 
tial preliminary of my general theory of physical force, and I am 
well aware that on this ground the truth of the theory must be 
contested. If this point be carried, the rest, I think, must 
follow. | 
There is already evidence from experiment that the action of 
physical force may be explained hydrodynamically. In the Phi- 
losophical Magazine for May (p. 348), Professor Maxwell has 
referred to a paper by M. Helmholtz on Fluid Motion, in which 
the author points out that lines of fluid motion are arranged 
according to the same laws as the lines of magnetic force. This, 
which Prof. Maxwell chooses to call a “ physical analogy,” I of 
course take to be confirmatory of the hydrodynamical theory of 
