When Janet Leigh finished her bread and butter Tony didn't want her to leave. Here was an attractive woman with nice skin sitting in his parlour talking to him as if he was a normal human man. This to him was love, and although he didn't know it, suddenly he was in it. And it might've been beautiful, except the next scene is the peephole scene.
Janet Leigh goes back to her room, and Tony stands, facing the camera. Hitchcock lights him from the side so that half his face is brightly lit and half his face is shadow. He swallows, listening to the footsteps walking into the room next door. That's when he walks over to the painting on the wall, lifts it off its nail and sets it on the floor. We see the small peephole notched in the wall and we see Tony looking in the peephole and we see Janet Leigh taking off her clothes. This time she's wearing a black bra and a black slip, and when the film cuts back to Tony's eye, watching, we can only imagine what he sees. All we know is that he's overcome.