He only told her she was imagining the lights dimming and then started making up new things like forgetting the mail or losing something to throw her off from inquiring further about the gaslights dimming.
-PJ
Yep. One of my favorite films (the Bergman/Boyer version).
He took advantage of her very observations about the dimming gas lights to exacerbate her self-doubt. Her distress was so extreme that she allowed him to persuade her that what she was in fact seeing was not in fact happening. She was trapped in his spider’s Web of malign deceipt. He was mentally torturing her.