734 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
whence 
Now whatever may be the inner meaning of this statement concerniDg the velocity 
of the internal ether, it certainly agrees with, and is at once suggested by, the fact, 
thoroughly established by both negative and positive experiments, that light travels 
down a running stream of matter at a pace 
-+^(l -K)- 
n \ / 
The negative experiments supporting this are such as the achromatic prism experi¬ 
ment suggested and tried by Arago, repeated more elaborately by Maxwell and by 
Mascart ; the water-telescope observations suggested by Boscovich, tried by Airy 
and by Hoek ; interference experiments of Babinet and of Hoek ; and several other 
experiments by Mascart. The positive experiment establishing it is the very 
beautiful and well-known one of Fizeau, now repeated and confirmed beyond the 
reach of any but cjuantitative cavil by Michelson.* 
Whether any ether is moved by moving matter may still be an open question, but 
that the speed of light is affected in a fairly ascertained way by the motion of 
transparent matter through which it is passing, is certain. 
8. But the specific motion of the internal ether is not the whole of Fresnel’s hypo¬ 
thesis ; there is the fixity of the external ether to be verified too. And that has not 
yet been done. In fact, one important experiment, to be discussed later on, throws 
grave doubt upon it, at least for large moving bodies like the earth. 
But unless the fixity of external ether be granted, our argument from density 
concerning the value of the internal velocity breaks down. Consider again tAVO 
planes moving with a slab of matter, one inside and one outside the mass, and let 
the space motion of the outside ether at the position of outside plane be affected by 
the motion of the slab to the extent yv, then all we can say is that 
v — yv — ir {v — X'v), 
or, 
a; = 1 — —~ 
7 (.“ ’ 
wherefore it is possible tor x and y to lie unity together. 
We may take it, however, that the quantitative accuracy of the Fizeau experi¬ 
ment renders anytliing of this sort very unlikely, and that Ave are bound to suppose 
the ether immediately outside moving matter to be stationary, i.e., to be com¬ 
pletely unaffected b}^ its motion, unless Ave are directly forced by facts to admit the 
contrary. 
* ‘ Amer. J. Sci.,’ vol. 31, p. 377. 
