It is not necessary to give up entirely even the ether. Many natural
philosophers find satisfaction in the idea of a material intermediate
substance in which the vibrations of light take place, and they
will very probably be all the more inclined to imagine such a medium
when they learn that, according to the Einstein theory, gravitation
itself does not spread instantaneously, but with a velocity that at
the first estimate may be compared with that of light. Especially in
former years were such interpretations current and repeated attempts
were made by speculations about the nature of the ether and about
the mutations and movements that might take place in it to arrive
at a clear presentation of electro-magnetic phenomena, and also of
the functioning of gravitation. In my opinion it is not impossible
that in the future this road, indeed abandoned at present, will once
more be followed with good results, if only because it can lead to the
thinking out of new experimental tests. Einstein's theory need not keep
us from so doing; only the ideas about the ether must accord with it.

Nevertheless, even without the color and clearness that the ether
theories and the other models may be able to give, and even,
we can feel it this way, just because of the soberness induced
by their absence, Einstein's work, we may now positively expect,
will remain a monument of science; his theory entirely fulfills
the first and principal demand that we may make, that of deducing
the course of phenomena from certain principles exactly and to the
smallest details. It was certainly fortunate that he himself put the
ether in the background; if he had not done so, he probably would
never have come upon the idea that has been the foundation of all
his examinations.

Thanks to his indefatigable exertions and perseverance, for he had