I cannot find an older Science Fiction short story about a shrinking scientist. It is not the story of Scott Cary in "The Shrinking Man" by Richard Matheson. It was a short story about a scientist that deliberately shrank himself knowing he would just continue to shrink and found a whole universe that waited for him to get small enough for them to contact him. Thank you.
I'm going to guess "He Who Shrank" (1936) by Henry Hasse. It involves a scientist inventing a shrinking solution and then finding civilized worlds within the microverses. What doesn't match is that it's the assistant who makes the journey, not the scientist.
Summary from TV Tropes:
The 1936 short story "He Who Shrank" ... a Mad Scientist injects his assistant with a serum that will make him shrink indefinitely. Said assistant shrinks to subatomic sizes, discovers that every atom is actually a solar system, lands on a planet, shrinks below the size of the atoms of that planet, and so on without end. It's a handy way of visiting innumerable planets without a spaceship
The end of the story has the protagonist visit our Earth.
When the aliens explain that they've been observing him:
The thought came again: "The scene has been speeded up a millionfold. The things you see taking place actually transpired over a great number of years; our scientists watched the phenomenon in great wonder, and many were the theories as to the cause of it. You are viewing yourself as you entered our nebula."
"So we watched and waited your coming for years, not knowing what you were or whence you came. We are still very much puzzled. You become steadily smaller, and that we cannot understand. We must hurry. Relax. Do not interfere with our process by trying to think back to the beginning, as you did before; it is all laid bare to us in the recesses of your brain. Simply relax, think of nothing at all, and watch the screen."
This was quite a popular idea in the pulp era of SF and I would guess lots of stories use it. Fuzzyboots mentions one such story, and another is Submicroscopic by Captain S. P. Meek. Both Submicroscopic and He Who Shrank are in Asimov's anthology Before the Golden Age.
In Submicroscopic the protagonist, Courtney Edwards, invents a shrinking machine and shrinks himself down to subatomic scale where we finds a new world including of course a beautiful princess whose life he saves and with whom he falls in love.