We are not completely certain what is happening here, and it has been the subject of much controversy for a very long time.
The poem’s author, William Blake, lived from 1757–1827. In “The Tyger”, he is using the same sort of rhyme that was earlier used by Alexander Pope (1688–1744) who in his “Essay on Man” once wrote a similar rhyme:
To Be, contents his natural desire;
He asks no angel’s wing, no seraph’s fire;
But thinks, admitted to that equal sky,
His faithful dog shall bear him company.
As Pope rhymed equal sky with company, Blake rhymed hand or eye with symmetry. Perhaps more importantly, Pope writes four lines of iambic tetrameter where the end rhyme differs in stress from what you would today expect. He does this again a bit later in the poem:
More rich, more wise; but who infers from hence
That such are happier, shocks all common-sense.
Heaven to mankind impartial we confess,
If all are equal in their happiness:
There Pope has rhymed we confess with happiness. That one differs only in stress, while the rhyme of equal sky with company differs (possibly) in that way but also in the quality of the vowel.
The [aɪ] and [ɪ] which we now think of as two completely separate vowels were once considered to be rhymes of each other. Another such example can be found in Pope’s “Epistle to Dr. Arbuthnot”, in which Pope rhymes lies and blasphemies (which is the same thing he did rhyming sky and company in his “Essay on Man”):
Or at the ear of Eve, familiar toad,
Half froth, half venom, spits himself abroad,
In puns, or politics, or tales, or lies,
Or spite, or smut, or rhymes, or blasphemies.
That was written only 60 years before Blake’s poem. Whether or not they actually “rhymed” then in the way that we think of rhyme, those two vowels were CONSIDERED rhymes at that time: Pope and Blake did not publish casual cacophony.
It is often observed how once upon a time, the last vowel of eye and sky was a long monophthong, the same one we now hear in tree. However, we CANNOT casually dismiss as a victim of the Great Vowel Shift the odd-to-us rhyming of Pope, let alone of Blake who lived after him, for sky was already a diphthong in Pope’s day, while symmetry was not.
It IS likely, however, that the final vowel of company and symmetry was a longer one in their days than we now find at the end of RP happy, which is now analysed as “short” in RP (although not so in North American analyses of Modern English).
One possibility is that there is a secondary stress on the last syllable of company, happiness, and symmetry. But even if so, we are stuck by the mismatch of a long vowel in eye with the short one ending symmetry. In H.P. Lovecraft’s essay on “The Allowable Rhyme”, the author writes that:
The rhyming of a long vowel with a short one is common in all the Georgian poets, and when well recited cannot but be overlooked amidst the general flow of the verse. . . .
Lovecraft discusses Pope at some length, observing:
Yet who can take offence? The unvarying ebb and flow of the refined metrical impulse conceals and condones all else.
This echoes the epigraph Lovecraft used at the top of his essay, a quotation from Horace* excusing the occasional formal lapse in Homer.
Pope was known to have rhymed not just obey with tea, but even also line with join, which no GVS explanation can possibly “explain”. Today we may call this slant rhyme, or even eye rhyme, but I am not so sure that was the intended effect, especially eye rhyme.
In the examples from Pope and Blake, the final foot differs from the predicted stress in a way that has an effect on the entire quatrain. The first three words have masculine rhymes, at least in assonance: bright – night – eye and desire – fire – sky. Then they (seem to?) switch to a different final foot where only the short vowel seems to match, and the stress not at all — maybe.
In Blake, assonant rhyme appears internally in the repeated Tyger, which is stressed, and in thy, which is not. He was setting up your ear. By asymmetrically breaking the pattern at the very end, he draws attention to it. The irony that the very symmetry of symmetry itself should be brought into question cannot be accidental.
So it may be that THAT was what he was doing. It might also be that he considered it a “close enough’ rhyme to fit the line. As Lovecraft observes, there is no question that our older poets had notions of rhyme that were not quite the same as our own.
Other examples of odd-to-us rhymes are easily found in Blake. For example, in “The Sick Rose”, he rhymes worm and storm. In most accents of Modern English, those two do not rhyme, although in some they do. What about in Blake’s accent? The real answer is that we’re not sure. They might have rhymed for him, but they may well not have.
If they did not, it is important that one not think them casual deviations, or misreadings. Their rarity speaks to this. Look but at the “perfect” rhyme of Blake’s “The Lamb”, the poem that became a Christmas carol, to see how strong his normal pattern was.
The occasional break from normal stress or rhyme can improve a poem, that it not become too plodding to the ear and so be reduced to trite doggerel. Certainly there are poets who casually ignore formal structure for no especial effect.
But Blake was no more one of these than was Pope, and yet both would from time to time deviate from obsessive precision in ways that elevate the form, not detract from it.
On the other hand, they may also have simply felt that [aɪ] and [ɪ] rhymed.
In the end, we cannot now be sure, howsoever many scholarly arguments should be produced.
The exact verse of Horace’s which Lovecraft was citing in his epigraph is the italic portion of the following longer extract from Horace’s Ars Poetica:
Sunt delicta tamen quibus ignovisse velimus.
Nam neque chorda sonum reddit, quem volut manus et mens,
poscentique gravem persaepe remittit acutum,
nec semper feriet, quodcumque minabitur arcus.
Verum ubi plura nitent in carmine, non ego paucis
offendar maculis, quas aut incuria fudit,
aut humana parum cavit natura. Quid ergo est?
Vt scriptor si peccat idem librarius usque,
quamvis est monitus, venia caret, et citharoedus
ridetur chorda qui semper oberrat eadem,
sic mihi, qui multum cessat, fit Choerilus ille,
quem bis terve bonum cum risu miror; et idem
indignor quandoque bonus dormitat Homerus.
Verum operi longo fas est obrepere somnum.
This was somewhat liberally but certainly artfully translated into English verse by Philip Francis (circa 1708–1773) as follows:
Yet there are Faults that we may well excuse,
For oft the Strings th’intended Sound refuse;
In vain his tuneful Hand the Master tries,
He asks a Flat, and hears a Sharp arise;
Nor always will the Bow though fam’d for Art
With Speed unerring wing the threatening Dart.
But where the Beauties more in Number shine,
I am not angry, when a casual Line
(That with some trivial Faults unequal flows.)
A careless Hand, or human Frailty shows.
But as we ne’er those Scribes with Mercy treat
Who, though advis’d, the same Mistakes repeat;
Or as we laugh at him who constant brings
The same rude Discord from the jarring Strings;
So, if strange Chance a Choerilus inspire
With some good Lines, with Laughter I admire;
Yet hold it for a Fault I can’t excuse
If honest Homer slumber o’er his Muse
And yet perhaps a kind indulgent Sleep
O’er Works of Length allowably may creep.
Poems like Pictures are, some charm when nigh,
Others at Distance more delight your Eye;
That loves the Shade, this temps a stronger Light,
And challenges the Critic’s piercing Sight;
That gives us Pleasure for a single View;
And this, ten times repeated, still is new.