57 
which there rests a still greater obscurity. If it really exists 
tbe knowledge of matter and of ether are plainly be the two 
pillars on which the science of physics must rest. But the 
doubts are greater, and the conflicts of opinion still more various 
than before. 
And first, does this ether exist? Such is the general 
opinion of physical students ; and for myself, I have no doubt 
ot its truth. But the dissentients are not few. M Comte 
denounces the theory as an equal illusion with the vortices of 
Bescartes. Mr. Lewes, his disciple, shares the same view. 
.Mr. Mill, in his Logic, inclines to the same side. The hypo- 
thesis, he says, is not without an analogy to that of Descartes, 
only that “ it is not entirely cut off from the possibility of 
direct evidence m its favour.” He has the strange idea that 
there can be some evidence of an hypothesis, besides that of its 
accounting for the phenomena it has to explain. Mr. Justice 
Drove, in his “ Correlation and Continuity,” holds strono-ly to 
the negative view. But the idea that the immensely diluted 
and attenuated matter of the planetary spaces can have the 
intense, elasticity implied by the speed of light seems to me 
wholly incredible. 
Next if ether exists, is it of one kind only, or more than 
one? By way of compensation to the last opinion, some 
theorists affirm that there are two kinds of ether, one called 
e A C i fcnC rr the ° ther luminous - Others go further. The authors 
of the Unseen Universe seem disposed to suggest a series of 
ethers, more and more subtile, of which the second rnav have 
nearly the same relation to the first which the first bears to 
common matter. This is very like a reproduction of the 
ceons and genealogies of the early Gnostics in a physical and 
material form. 
Again, is the ether continuous, or discontinuous and atomic ? 
Professor Challis seems to me to hold strongly the former, 
bffi Newton, Young, Fresnel, Airy, Cauchy, Stokes, and most 
other physical philosophers, the latter view. 
Is this ether attractive or self-repulsive ? The latter, the 
usual opinion, seems to me essential to a just conception of its 
nature But Professor Bayma, in his Molecular Physics, main- 
tains that it must be attractive. And Sir George Airy, in 
private, once told me that, in his opinion, the phenomena of 
light require the notion of attractive or contractile forces, and 
stretched strings, rather than repulsive force-centres, though 
this must imply some kind of fastening or attachment to walls 
of the universe. 
Again, what is the relation between ether and common 
