ethan reveals his suspicion about zeena's illness. what is it?

jealousy......

"...his wife always seemed to be asleep when he left her side in the winter darkness, and he had stupidly assumed that she would not notice any change in his appearance. Once or twice in the past he had been faintly disquieted by Zenobia's way of letting things happen without seeming to remark them, and then, weeks afterward, in a casual phrase, revealing that she had all along taken her notes and drawn her inferences. Of late, however, there had been no room in his thoughts for such vague apprehensions. Zeena herself, from an oppressive reality, had faded into an insubstantial shade."