According to Helen Stoner's long description of the night Julia Stoner died two years earlier, Helen was holding a burned out match in one hand and a matchbox in the other. These are the exact words:
“Was your sister dressed?”
“No, she was in her night-dress. In her right hand was found the charred stump of a match, and in her left a match-box.”
“Showing that she had struck a light and looked about her when the alarm took place. That is important."
Evidently the deadly snake had been on her bed for three nights in a row before the evil Dr. Roylott summoned it back with a whistle at about three o'clock each morning. On the fourth night Julia must have finally been bitten. She could have been bitten after she lit the match, but it seems more likely that she heard the whistle, been bitten while reaching for the matchbox, and then lit the match. By the dim matchlight she saw something that looked to her like a speckled band slithering up the dummy bell-pull. By that time she was already dying and in a delirium.
The author could not have Julia tell Helen that she had seen a snake crawling up the bell-pull, because that would have given the stepfather's sinister plot away and spoiled the whole story. We do not learn until much later that Roylott kept a deadly poisonous swamp adder in a safe in his room and was using it for murder.
The room both Julia and Helen slept in was pitch-dark because they always closed and bolted the shutters. There was, of course, no electricity in those days. They had to light a candle or a lamp, and that took a bit of time. Julia must have heard the whistle, made a quick movement to find her matches, and at that point had been bitten by the snake. Holmes and Watson could see nothing when they were hiding in the dark room until Holmes struck a match.
The instant that we heard it, Holmes sprang from the bed, struck a match, and lashed furiously with his cane at the bell-pull.
“You see it, Watson?” he yelled. “You see it?”
The author, Sir Arthur Conan Doyle, does not say so--perhaps because of Victorian inhibitions--but it seems likely that the snake actually crawled under the bedcovers for warmth on each of the four nights it spent with Julia, as well as on the one night it spent with Helen. Early in the story the author makes it clear that the weather is very cold. The swamp adder comes from a tropical climate and would seek warmth rather than trying to escape into the outdoors. We can only imagine the horrible images of a poisonous snake crawling under the covers with a sleeping girl and curling up right alongside her body for warmth. The snake would not bite the girl as long as she did nothing to alarm or provoke it. But on the night that Julia got bitten, she must have rolled over in order to reach for the matchbox and rolled right on top of the snake. It would have bitten her through her nightgown, so no bite mark would be visible. Dr. Roylott must not have expected the snake to kill Julia on that particular night. He was recalling it, as usual, at three o'clock because he thought it hadn't done its deadly work. Helen must have had that same snake in bed with her on the night she heard the whistle. She tells Holmes and Watson that she sprang up and lit the lamp but saw nothing. The snake must have already left the bed and Helen didn't notice it climbing up the bell-pull. If she hadn't come to see Holmes the next morning, the snake would have killed her eventually.