the invisible ray be part science fiction and part horror . it be also the third of seven karloff or lugosi feature . in this entry the dominant role go to boris karloff . through specially design astronomical equipment and dr. janos rukh ( karloff ) demonstrate to colleague dr. felix benet ( bela lugosi ) and sir france stevens ( walter kingsford ) and his wife lady stevens ( beulah bondi ) and their nephew ronald drake ( frank lawton ) and that a meteorite contain a powerful element and land on the african continent many million of year in the past . rukh impress his guest who invite him to accompany them on an expedition to africa to find the mysterious element . rukh go off on his own and discover the place where the meteorite land and the element which he name radium x. due to rukh long absence and his comely young wife diana ( france drake ) becomes attract to drake and the two fall in love . meanwhile rukh becomes contaminate by radium x to the point that anyone he touch will die instantly . the contamination cause his skin to emit a bright glow in the dark . rukh go to benet for help . benet devise an antidote which if take on a daily basis and will provide temporary immunity to the element . unfortuneatly and the deadly element also affect rukh brain and slowly turn him into a vindictive murderer . while rukh be continue his work and sir france take news of the discovery back to france . diana and ronald accompany him . when benet inform him of this action and rukh accuse the party of betray him and his discovery and secretly plan his revenge . when he return to france he learn of the heal power of radium x a he cure his mother ( violet kemble cooper ) blindness and of benet work in cure the malady of his patient . but rukh madness intensifies . first he murder an innocent man whom be identify a rukh . upon hearing of rukh apparent demise and diana and ronald marry . this anger rukh and he begin to exact his revenge on the five other member of the expedition . one night he be lure into a trap set by benet and . karloff be excellent in the lead role move from a happily marry ambitious scientist to a raving maniac . lugosi have a straight role for once and do what he can with the limited part . at 79 minute this film be the second longest of the six karloff or lugosi collaboration . karloff film all the way .