In other words, is the ether wholly or partially stagnant near the earth, or is it streaming past us with the opposite of the full terrestrial velocity of nineteen miles a second? Surely if we are living in an ether stream of this rapidity we ought to be able to detect some evidence of its existence.[4] It is not so easy a thing to detect as you would imagine. We have seen that it produces no deviation or error in direction. Neither does it cause any change of colour or Doppler effect; that is, no shift of lines in spectrum. No steady wind can affect pitch, simply because it cannot blow waves to your ear more quickly than they are emitted. It hurries them along, but it lengthens them in the same proportion, and the result is that they arrive at the proper frequency. The precise effects of motion on pitch are summarised in the following table:-- _Changes of Frequency due to Motion._ Source approaching shortens waves. Receiver approaching alters relative velocity. Medium flowing alters both wave-length and velocity in exactly compensatory manner. * * * * * What other phenomena may possibly result from motion? Here is a list:-- _Phenomena resulting from Motion._ (1) Change or apparent change in direction; observed by telescope, and called aberration. (2) Change or apparent change in frequency; observed by spectroscope, and called Doppler effect. (3) Change or apparent change in time of journey; observed by lag of phase or shift of interference fringes. (4) Change or apparent change in intensity; observed by energy received by thermopile. * * * * * What we have arrived at so far is the following:-- Motion of either source or receiver can alter frequency; motion of receiver can alter apparent direction; motion of the medium can do neither. But the question must be asked, can it not hurry a wave so as to make it arrive out of phase with another wave arriving by a different path, and thus produce or modify interference effects? Or again, may it not carry the waves down stream more plentifully than up stream, and thus act on a pair of thermopiles, arranged fore and aft at equal distances from a source, with unequal intensity? And once more, perhaps the laws of reflection and refraction in a moving medium are not the same as they are if it be at rest. Then, moreover, there is double refraction, colours of thin plates and thick plates, polarisation angle, rotation of the plane of polarisation; all sorts of optical phenomena that need consideration. It may have to be admitted, perhaps, that in empty space the effect of an ether drift is difficult to detect, but will not the presence of dense matter--especially the passage through dense transparent matter--make the detection easier? So a great number of questions arise, all of which have been, from time to time, seriously discussed. _Interference._