This was a good episode as you knew all the characters and their motives from the start and therefore there weren't any real surprises.  The one female student was a little too cliché, but still a character for the story.

Chas - Speculum is what you found in your researches, but further research would've found it is also a type of metal used in mirrors for telescopes and some mirrors (usually about 2/3 copper and 1/3 tin + a little bit of arsenic).