Unusual usage of "do" in "do you follow your path", in a sentence that is not a question

I don't understand the structure of this sentence:

“Well, well, Inspector,” said he. “Do you follow your path and I will follow mine.

It is from The Adventure of Wisteria Lodge, part 2, a Sherlock Holmes short story.

Why is there a "do" there? shouldn't it be "you follow", or "you do follow"?

This is a common construction in older English. It is an imperative, and I believe do used to be required (or very common) when the subject (= "you") of an imperative is expressed; constructions like you tell me! (with omitted do) sound modern to my ear, and often informal. I know it was at least still common in late-19th-century literature (Trollope's The Way We Live Now, for example).