history of - What was the earliest SF work that used the idea of the "Multiverse"?

The discussion below this question leads me to ask: What was the earliest SF work that used the idea of the Multiverse (parallel universes or alternate worlds)? I'm looking for fairly hard SF treatments, where the idea is presented as a real, physical possibility rather than magical or fantasy worlds, such as Narnia.

Answer

1915: A Drop in Infinity, a novel by Gerald Grogan, available at the Internet Archive. Reviewed by Everett F. Bleiler in Science-Fiction: The Early Years:

A robinsonade in the fourth dimension.

Jack Thorpe and Marjorie Matthews are walking along the shore in Cornwall when a seeming eccentric asks them directions. They humor him by showing the way, whereupon he produces a revolver and takes them captive. A scientist who has worked in dimensional research, he is brilliant, but unfortunately mad and irresponsible. He thinks of himself as a hubble-bubble, and so the characters call him.

The Hubble-Bubble reveals that he has obtained access, via the fourth dimension, to two other worlds which in modern terminology amount to parallel worlds. His technique involves electricity, vibrations, and a mental set. He now offers Jack and Marjorie the choice of death or entry to another world, which he claims is much like earth in fauna and flora, but without human or other intelligent life.

Jack and Marjorie have little choice, and in a short time find themselves in the world they later call Marjorie-land. Making the best of the situation, they work out a Crusoe-like primitive culture, building a house, cultivating certain plants, and domesticating animals. From time to time a few other humans are dropped in with them, a total of four batches in all. Most of them are congenial, but Michael Quelch, a lazy, vicious Cockney will eventually cause trouble.

On one occasion the Hubble-Bubble's apparatus seems to have "backfired," and Jack is temporarily returned to our world. But he makes terms of a sort with the mad scientist and goes back to Marjorie-land.

Time passes. Jack and Marjorie have two children, and the colonists thrive. Life seems reasonably secure and happy. But then Quelch causes trouble. Thinking that Jack is dead when he does not return on time from a journey of exploration, Quelch tries to seize control of the settlement, rape Marjorie, and murder the children. Fortunately, as in a novel by Edgar Rice Burroughs, Jack returns in the nick of time. After some complications Jack reluctantly sets out to hunt Quelch down and kill him, but Quelch is found dead of natural causes.

After a time it becomes apparent that the Hubble-Bubble is also dead and that the small human colony in Marjorie-land is permanently isolated.

The story is told by Thorpe, a generation or two later.

In the first part of the book, the treatment is flippant, but the story soon settles down to a rather dull development. Of historical interest as a very early parallel worlds story.