I realise that Blake isn't easily appreciated by mere mortals, but why did he spell the word "tyger" rather than "tiger"? The poem was published 50 years after Johnson's dictionary.
I'm probably missing something, as usual.
Johnson:
Ti'ger. n.s. [tigre, Fr. tigris, Latin.] A fierce beast of the leonine kind.
Shakespeare:
When the blast of war blows in your ear,
Then imitate the action of the tiger:
Stiffen the sinews, summon up the blood.