PARIS: You didn’t take one note. You resorting to the osmosis theory of learning?
RORY: Why do you care?
PARIS: I don’t, just making an observation.
RORY: Great, we’ll build a dome over you and jam a telescope in your head.
In science, osmosis is the movement of solvent molecules through semi-permeable membrane; it’s a vital process in biological systems, and is the primary means by which water is transported in and out of cells.
Social osmosis is the indirect infusion of social or cultural knowledge without direct knowledge – an example would be knowing all about the latest blockbuster movie, such as the plot, setting, all the characters and actors playing them, without ever actually watching the movie. Paris seems to be suggesting that this is how Rory is planning to learn about Henry James.
Rory’s retort is saying that if Paris is going to make observations, she can have a dome and a telescope – like an astronomical observatory.