People didn't believe at first that William Herschel could see the things he was claiming to pick out in the sky, but then his speculum mirror technology was so in advance of anything that had been produced at that stage.
When Rachel, as part of the plan, enrolls as a patient, and lies back in front of him with her feet in stirrups, awaiting the speculum, we sense a prickle of wariness, and a prod of menace, in every question he asks.