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ONANEWVARIETYOFOCULARSPECTRUM.ByJOHNAiTKEN.IFwelookforashorttimeatanyobject,andafterwardsturntheeyeinanotherdirection,weseeaspectralimageoftheformoftheobjectfirstlookedat.Again,ifafterwehavelookedatanycolouredobjectweturntheeyeinanotherdirection,weseeaspectralimageofacolourcomplementarytothatfirstlookedat.Inadditiontothesespectralformsandcolours,Ifindthereisanotheranddistinctkindofocularspectrum,whichwemaycallamotionspectrum.Itisseenwhenwelookfirstatabodyinmotionandafterwardsdirecttheeyetowardsanobjectatrest.Theobjectatrest,whenseenundertheseconditions,seemstobeinmotion,andthedirectionofitsapparentmotionistheoppositeofthatofthemovingbodyfirstlookedat.Ifirstobservedthismotionspectrumwhenlookingatthesurfaceofariverwhereitwasflowingrapidly,theeyebeingafterwardsdirectedtoagravelbank.Thefirsteffectseemedtobeanindistinctnessofvision,but,oncarefullyrepeatingtheexperiment,Iwasmuchastonishedtoobservethat,afterlookingsteadilyatthestreamandthenatthegravelbank,anarrowspectralstreamofgravelseemedto...