He falters. History and classics, that's exactly the sort of combination that could see through his lie. Whether it's considered fact, or only a work of fiction. There's only a single instant in which his face falls, into an even more brief flash of fear. But Jekyll picks himself back up quickly. It's so much worse to show it.

"It is not so complicated a matter as it may seem! I am certain you would find the sciences really quite agreeable if you were to give them a go. History and literature are ever expanding in much the same way, after all. I'm afraid I have had little opportunity for such subjects myself."