I hate to appear the 'pleb' in this discussion, but ...
As I'm British (decidedly not English) I only recognise this through its being a quotation used in "The Dead Poets Society".

I'm sorry if I see no connection with the assassination of Lincoln. A quick check on 'Wiki' says it's an extended metaphor. But if you don't know it's supposed to be about Lincoln. there is no clue available!

Thanks Mmario for the full quote. That extends my reading a little.