Tyger Tyger, burning bright,
In the forests of the night;
What immortal hand or eye,
Could frame thy fearful symmetry?
It is, he says, a puzzle that has nagged at him for decades. Should it rhyme or not?
It's not just him. The couplet has generated quite a lot of ink. Some commentators say it is simply an eye-rhyme. That to my mind, is the lazy solution. At a time when the spelling system was becoming standardized, eye-rhymes were certainly a possibility, and some poets used them a lot, but why have just one eye-rhyme in a poem where all the other rhymes are exact phonological partners?
To my mind a phonological explanation is much more likely. Blake is recalling an earlier pronunciation of final -y which did in fact rhyme with words like eye. A classic example is Oberon's speech in A Midsummer Night's Dream (3.2.102), where we find dye, eye, espy, sky, and by intercalating with archery, gloriously, and remedy. Eye rhymes are not a viable explanation in Shakespeare's time, as the spelling system wasn't stable enough to guarantee them, and the writers on pronunciation at the time (the orthoepists) are always stressing the auditory value of rhyme.
Some of these writers actually try to capture the phonetic quality of those -y words. John Hart, for example, writing in the 1570s, transcribes boldy as boldlei, certainly as sertenlei, and so on. Clearly he is trying to convey some sort of diphthong here. In my work on Shakespearean Original Pronunciation, I transcribe this as a schwa + i. It is a pronunciation which lasted into the 18th century in educated speech, and of course can still be heard in some regional accents today.
By the time Blake was writing, the everyday pronunciation had shifted to its modern form, like a short 'ee'. This is how John Walker, for example, describes symmetry in his Pronouncing Dictionary, published just three years before Blake's poem: he rhymes it with me. The pronunciation with the final diphthong would have sounded distinctly old-fashioned by then. But wouldn't that suit someone who begins a poem with a spelling of tyger that was also archaic?
The phonaesthetics of the stanzas adds further support for a phonological rather than an orthographic explanation. That schwa + i diphthong turns up in Tyger, and is then echoed at the end of all eight lines of the opening two stanzas: bright, night, eye, symmetry, skies, eyes, aspire, fire. To my ear, this adds the same kind of mystical atmosphere that we hear when Oberon's speech is read in an OP way.
People do remember long-gone pronunciations. If I pronounce the word 'lord' as 'lahd', perhaps in a parody of upper-class speech, I am actually producing the normal pronunciation of this word as it was a century ago. Daniel Jones, writing in the 1910s, locates it in the place where today (in RP) we would find the vowel of far. Nobody says 'lahd' any more, but if I were to write a rhyming poem in which I had the following lines, I think readers would have no difficulty in 'hearing' the old pronunciation despite the modern spelling:
The butler looked all round the yard:
'There's no-one in the grounds, my lord'.
Some writers would opt for a nonstandard spelling here (such as 'lahrd') but such alternatives are not often available in the orthography - as in the case of symmetry. There was no archaic spelling for Blake to fall back on, in this case, to help the reader. And so we have the spelling that has come down to us.