The apostrophe comes at the very end only when the s has been added to a word to make it plural. You don't add an s to make a word plural and another s to make

The apostrophe comes at the very end only when the s has been added to a word to make it plural. You don't add an s to make a word plural and another s to make it possessive. If you write about the wealth of Donald Trump, it's "Trump's money" but the wealth of the Trump family (i.e. multiple people) it's "Trumps' money." There is no singular "Fitt" so Fitts's Law is correct, same as "Laura Poitras's films." The apostrophe also comes at the very end when "es" is added to make a word plural, e.g. "fishes' spawning ground."