He drank out of the bottle till he saw the island; the island is the rising bottom of a wine bottle, which appears like an island in the centre, before the bottle is quite empty.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Ironmonger’s Shop * IvoriesNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes