Then he saw that it was indeed so arranged, and that
his daughter was clad in a sort of loose, extremely long
knickerbockers

Conan Doyle calls a divided skirt knicerbockers. The garment by that name that was briefly popular in the twenties was similar to old time Dutch trousers, which
were loose below the knees, and had a knit cuff, and stockings were knee length. Golfers wore ones called "plus fours" which came well below the knee. I have seen them for a very long time.

Here is URL with a picture of can of Ductch Boy Paint.
Scroll down to fourth picture:
http://memory.loc.gov/ammem/ndlpedu/features/branding/did_you_know.html

The baggy pants he wears were origin of knickerbockers,aka knickers.