1892.] on the Motion of the Ether near the Earth. 573 



What other phenomena may possibly result from motion ? Hero 

 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 appareut change in intensity ; observed by energy received by 

 thermopile. 



Motion of either sourcy or receiver can alter frequency, motion of 

 receiver can alter apparent direction, motion of the medium can do 

 neither ; but surely it can 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, it may 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 again, 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. 



It may be, perhaps, that in empty space the effect of an ether 

 drift is difficult to detect, but will not the presence of dense matter 

 make it easier ? 



Consider No. 3 of the phenomena tabulated above. I expect 

 that every one here understands interference, but I may just briefly 

 say that two similar sets of waves " interfere " whenever and 

 wherever the crests of one set coincide with and obliterate the 

 troughs of the other set. Light advances in any given direction when 

 crests in that direction are able to remain crests, and troughs to 

 remain troughs. But if we contrive to split a beam of light into two 

 halves, to send them round by different paths, and make them meet 

 again, there is no guarantee that crest will meet crest and trough 

 trough ; it may be just the other way in some places, and wherever 

 that opposition of phase occurs there there will be local obliteration 

 or " interference." Two reunited half-beams of light may thus 

 produce local stripes of darkness, and these strij^es are called inter- 

 ference bands. 



If I can I will produce actual interference of light on the screen, 

 but the experiment is a difficult one to make visible at a distance, 

 partly because the stripes or bands of darkness are usually very 

 narrow. I have not seen it attem^ tod before. [Very visible bauds 



