"Yes, this is it. He gave him a dose of strychnine. And Keppel took it. So far as I can judge, they've been dead about the same length of time."
She had begun to tremble. "Put it down, Ken! Put it down! We've left finger-prints all over the place. Seriously — don't you think we'd better get out of here? Did you get the envelope you came for? The one in the desk?"
"Yes."
"Then that's all we're supposed to do. If they catch us here now…" She stopped, for she could not let go the puzzle. "I say, are there-are there any cuff-links on the desk, or any books missing, or anything like the arrangement you saw in the other place?"
We went back to the table. This work-table was as untidy as Hogenauer's had been clean. There were little bundles of note-sheets, scribbled with mathematical formula; and (to me) similarly cryptic markings; I presumed they were notes for Keppel's physics lectures. There were books with notesheets thrust into them to keep the places, and several coloured pencils. But all these had been pushed aside to make a clear path down the middle of the table. In this cleared space lay a flattish piece of glass, some three inches in diameter. Its underside was flat, its top very slightly convex. It lay against a bronze ash-tray in which were the stumps of many cigars. Close to it, on one of the note-sheets, symbols had been idly scrawled with a blue pencil. Thus:
If a' be the angle of refraction, and t the thickness of the plate, then
BC cos a’ = t
BD = 2BC sin a’ = 2t tan a’
2 µBC — Bd = 2tµ cos a'
"It's something," said Evelyn, "to do with light, or the refractions of light, or got it! I know what that piece of glass is, anyhow. It's the lens of a child's magic-lantern."