In Shakespeare's time, because of the [Great Vowel Shift](http://en.wikipedia.org/wiki/Great_Vowel_Shift), *symmetry* was a much closer rhyme with *eye* than it is today (if it wasn't exact), and Shakespeare and his contemporaries used rhymes like this all the time.

Shakespeare: Sonnet 1:

> From fairest creatures we desire increase,  
That thereby beauty's rose might never die,  
But as the riper should by time decease,  
His tender heir might bear his memory:

Shakespeare: Sonnet 33:

>  Full many a glorious morning have I seen  
Flatter the mountain-tops with sovereign eye,  
Kissing with golden face the meadows green,  
Gilding pale streams with heavenly alchemy;

By Blake's time, I suspect that the Great Vowel Shift had progressed enough that these words no longer rhymed well, but Blake (along with some of his contemporaries) seems to have thought that the fact that Elizabethans rhymed these words gave him the license to do so as well.