730 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
to interstellar or free ether. When I speak of the ether anywhere as “free,” I mean 
that its properties are identical with the interstellar ether enormously distant from 
all gross matter. And this is the condition of ordinary space, except for the presence 
of meteoric particles, whose influence, if any, we at present legitimately ignore. 
The only hypothesis which at first sight appears to assume infinitely distant ether 
to be affected by the motion of, say, the earth, is that of Sir George Stokes, in 
1845, where an irrotational motion, zero with respect to the earth, was postulated for 
it. But he must have seen some way in which so impossible an assumption could be 
avoided ; and the question how far any kind of irrotational motion can be conceived 
of as allowing rest at infinity, and yet no slip at the earth’s surface, will be discussed 
later, § 31. 
(ii.) Inside material bodies the ether is modified. —We learn this by direct experi¬ 
ment and observation. 
For transparent bodies we learn it by optical experiments, which proves that light 
travels more slowly through their modified ether than it does in free ether; while at 
the same time there is no doubt but that the ether inter-penetrates them, because 
material substance itself is wholly incompetent to transmit anything possessing the 
properties and the speed of radiation. 
In metallic bodies we find great opacity combined with anomalous dispersion and 
otlier complex efiects. In them evidently the ether is intensely modified, if it exists 
at all. 
I shall call the ether inside gross matter of any kind “ modified ether,” but as to 
the particular way it is modified I make no assumption. [Electrostatic experiments 
suggest that inside transparent bodies, something which may be called its “ virtual 
elasticity ” is diminished. Magnetic experiments suggest that inside several opaque 
substances it is loaded, so as to increase what may be called its “virtual density;” 
and there is a temptation to identify Itt/K with the one, and Itt/x with the other, of 
these two ethereal constants. Further, electrokinetic experiments suggest that inside 
metallic conductors the ether has a virtual viscosity, whereby its motion through 
matter is resisted precisely as the first power of the velocity. But none of these 
doubtful hypotheses shall here be obtruded.] 
Rate of Travel qf“ Modified ” Ether. 
3. Defined in this way it is quite obvious that “modified ether ” travels at the same 
steady pace as its material encasement. For lift a lump of glass or of copper from 
one side of a table to the other, the modified ether which was in one place is now in 
another, and has necessarily accompanied the material body. If tJie modification of 
eflier by matter requires time, there woidd be some lag during epochs of acceleration ; 
but during steady velocity there would even so be no difference in speed between 
modified ether and matter, only a slight lag in space. 
