The Dark Other by Stanley G. Weinbaum

Pat awoke in rather better spirits. Somehow, the actual entrance of Dr. Horker into the case gave her a feeling of security, and her natural optimistic nature rode the pendulum back from despair to hope. Even the painful black-and-blue mark on her arm, as she examined it ruefully, failed to shake her buoyant mood.

Her mood held most of the day; it was only at evening that a recurrence of doubt assailed her. She sat in the dim living room waiting the arrival of her mother's guests, and wondered whether, after all, the predicament was as easily solvable as she had assumed. She watched the play of lights and shadows across the ceiling, patterns cast through the windows by moving headlights in the street, and wondered anew whether her faith in Dr. Carl's abilities was justified. Science! She had the faith of her generation in its omnipotence, but here in the dusk, the outworn superstitions of childhood became appalling realities, and some of Magda's stories, forgotten now for years, rose out of their graves and went squeaking and maundering like sheeted ghosts in a ghastly parade across the universe of her mind. The meaningless taunts she habitually flung at Dr. Carl's science became suddenly pregnant with truth; his patient, hard-learned science seemed in fact no more than the frenzies of a witch-doctor dancing in the heart of a Rhodesian swamp.