In order for most of Hitchcock's subjective camerawork to be successful, he relies on signs. For example, in both Rear Window and Psycho, Hitchcock uses shots that have vignette around the sides. To the audience, this can easily suggest that we are viewing the images through some sort of mediated device besides the camera, like a peephole or binoculars.
He relies on the audience having some sort of experience with viewing through binoculars or something similar. What he is referring to is a more abstract visual experience than a simple signified object. He calls upon also looking through an object, but also what that sort of experience means. By aligning that visual reference with the film, he is also aliging the voyeuristic experience with viewing in the film.