ams ▁are ▁made ▁to ▁follow ▁a ▁traject ory ▁in ▁opposite ▁directions . ▁On ▁return ▁to ▁the ▁point ▁of ▁entry ▁the ▁light ▁is ▁allowed ▁to ▁exit ▁the ▁platform ▁in ▁such ▁a ▁way ▁that ▁an ▁inter ference ▁pattern ▁is ▁obtained . ▁La ue ▁calculated ▁a ▁disp lacement ▁of ▁the ▁inter ference ▁pattern ▁if ▁the ▁platform ▁is ▁in ▁rotation   – ▁ ▁because ▁the ▁speed ▁of ▁light ▁is ▁independent ▁of ▁the ▁velocity ▁of ▁the ▁source , ▁so ▁one ▁beam ▁has ▁covered ▁less ▁distance ▁than ▁the ▁other ▁beam . ▁An ▁experiment ▁of ▁this ▁kind ▁was ▁performed ▁by ▁Georges ▁S agn ac ▁in ▁ 1 9 1 3 , ▁who ▁actually ▁measured ▁a ▁disp lacement ▁of ▁the ▁inter ference ▁pattern ▁( S agn ac ▁effect ). ▁While ▁S agn ac ▁himself ▁concluded ▁that ▁his ▁theory ▁confirmed ▁the ▁theory ▁of ▁an ▁a ether ▁at ▁rest , ▁La ue ' s ▁earlier ▁calculation ▁showed ▁that ▁it ▁is ▁compatible ▁with ▁special ▁relativ ity ▁as ▁well ▁because ▁in ▁both ▁theories ▁the ▁speed ▁of ▁light ▁is ▁independent ▁of ▁the ▁velocity ▁of ▁the ▁source . ▁This ▁effect ▁can ▁be ▁understood ▁as ▁the ▁elect romagnet ic ▁counter part ▁of ▁the ▁mechan ics ▁of ▁rotation , ▁for ▁example ▁in ▁anal ogy ▁to ▁a ▁Fou ca ult ▁p end ul um . ▁Al ready ▁in ▁ 1 9 0 9 – 1 1 , ▁Franz ▁Har ress ▁( 1 9 1 2 ) ▁performed ▁an ▁experiment ▁which ▁can ▁be ▁considered ▁as ▁a ▁synth esis ▁of ▁the ▁experiments ▁of ▁F ize au ▁and ▁S agn ac . ▁He