Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
Describing Shade's murder by Gradus, Kinbote (in VN’s novel Pale Fire, 1962, Shade’s mad commentator who imagines that he is Charles the Beloved, the last self-exiled king of Zembla) quotes a line from Matthew
Arnold's poem The Scholar-Gipsy (1853):