What does Holmes deduce about the footprints on the windowsill? Part 1

After questioning Mrs. Douglas, Holmes muses for a moment and asks Ames to come in. He inquires what shoes Barker was wearing the night of the murder. Ames responds that he was wearing house slippers. Holmes finds the slippers and looks on the bottom. They are covered in blood, and the mark of the window matches that of the sole of the slipper. The men are all astonished.