What is the solution in The Adventure of Speckled Band?

In The Adventure of the Speckled Band, Holmes deduced that Dr. Grimesby Roylott was using a venomous swamp adder fastened to a bell-cord pulley to kill his stepdaughter, Julia, who was locked up in the room, and that Julia's twin sister, Helen Stoner, was also exposed to the snake the night their stepfather was found dead in his room, and that was the reason she was so distraught. To solve the case, he had Watson stay with Julia in the room so as to provide a presence within the room, while Holmes himself waited outside. When Dr. Roylott attempted to release the snake down the vent to kill Julia once again, Holmes was there to kill it just in time and expose Dr. Roylott's villainous plot. Dr. Roylott died after suffering a heart attack.