Fizeau also suggested, but did not attempt, what seems an easier experiment, with fore and aft thermopiles and a source between them, to observe the drift of a medium by its convection of energy; but arguments based on the law of exchanges[5] tend to show, and do show as I think, that a probable alteration of radiating power due to motion through a medium would just compensate the effect otherwise to be expected. We may summarise most of these statements as follows:-- _Summary._ { A real and apparent change of wave-length. { Source alone { A real but not apparent error in direction. moving produces { { No lag of phase or change of intensity, { except that appropriate { to altered wave-length. { No change of frequency. Medium alone { No error in direction. moving, or { A real lag of phase, but undetectable source and receiver { without control over the moving { medium. together, produces { A change of intensity corresponding { to different distance, { but compensated by change { of radiating power. { An apparent change of wave-length. { An apparent error in direction. Receiver alone { No change of phase or of intensity, moving produces { except that appropriate { to different virtual { velocity of light. I may say, then, that not a single optical phenomenon is able to show the existence of an ether stream near the earth. All optics go on precisely as if the ether were stagnant with respect to the earth. * * * * * Well, then, perhaps it _is_ stagnant. The experiments I have quoted do not prove that it is so. They are equally consistent with its perfect freedom and with its absolute stagnation; though they are not consistent with any intermediate position. Certainly, if the ether were stagnant nothing could be simpler than their explanation. The only phenomena then difficult to explain would be those depending on light coming from distant regions through all the layers of more or less dragged ether. The theory of astronomical aberration would be seriously complicated; in its present form it would be upset (p. 45). But it is never wise to control facts by a theory; it is better to invent some experiment that will give a different result in stagnant and in free ether. None of those experiments so far described are really discriminative. They are, as I say, consistent with either hypothesis, though not very obviously so.