Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
In Canto Three of his poem John Shade (the poet in VN’s novel Pale Fire, 1962) describes his heart attack (during which he saw a tall white fountain) and mentions Captain Schmidt and Captain Smith:
If on some nameless island Captain Schmidt
Sees a new animal and captures it,
And if, a little later, Captain Smith
Brings back a skin, that island is no myth. (ll. 759-762)