Tarzan and the Jewels of Opar by Edgar Rice Burroughs

Sleepless upon his blankets, Albert Werper let his evil mind dwell upon the charms of the woman in the nearby tent. He had noted Mohammed Beyd's sudden interest in the girl, and judging the man by his own standards, had guessed at the basis of the Arab's sudden change of attitude toward the prisoner.

And as he let his imaginings run riot they aroused within him a bestial jealousy of Mohammed Beyd, and a great fear that the other might encompass his base designs upon the defenseless girl. By a strange process of reasoning, Werper, whose designs were identical with the Arab's, pictured himself as Jane Clayton's protector, and presently convinced himself that the attentions which might seem hideous to her if proffered by Mohammed Beyd, would be welcomed from Albert Werper.