A William Butler Yeats Poem

This great purple butterfly,
In the prison of my hands,
Has a learning in his eye
Not a poor fool understands.

Once he lived a schoolmaster
With a stark, denying look,
A string of scholars went in fear
Of his great birch and his great book.

Like the clangour of a bell,
Sweet and harsh, harsh and sweet,
That is how he learnt so well
To take the roses for his meat.