That cagèd heart makes iron stroke, I do not love you now the less, Or less that all that light once gave The graduate dark should now revoke
If this is the speaker's idea of a declaration of love, then be still our Shmoopy heart. In some ways, the "cagèd heart" is always in a cage (the rib cage), but put this way, the accommodations feel all the more close and constricting.
(Note: that funky accent in "cagèd" means that you should add a syllable to the word and put a stress on that second syllable: caged. Check out "Form and Meter" for more on that.)
The notion of an "iron stroke" is a tricky one. A "stroke" can be a gentle gesture, like a stroke on the cheek. It can also be a quick happening, like a stroke of genius or a stroke of luck. We also think of it as a forceful motion, like the stroke of an oar pulling a boat or the stroke of a piston in an engine. Probably, the speaker's not talking about a medical stroke here—or Different Strokes for that matter.
Likely, it's the idea of a forceful motion that the speaker is trying to get across. Since it's paired with "iron" and in the context of a cage, we're imagining a heart that has turned aggressive and forceful, rather than kind and tender. Any lovey-dovey stroking can be forgotten when things get all cage-y and metallic.
All the same, our speaker seems like a forgiving chap. He "do[es] not love you now the less." Wait—that's good, right? Well, it's not bad, exactly. Let's be clear: he's not saying that he loves his beloved more, only "not less," even though she's apparently got some ex-con heart that is all tough and mean. There's something his beloved can write in her diary framed in pink hearts.
What's more, or what's not less, is this love, now that what was given by light has been rejected by darkness (come on, darkness, get it together). Look at the word choice here: "graduate" is used unusually, as an adjective. And yes, it does mean by degree, but it also evokes a certain progress to a new level. We wonder if that darkness is getting worse.
Whatever it's doing, it's telling everything that the light gave us to "talk to the hand." "Revoke," which rhymes so nicely with "stroke," is a word from the field of law, meaning to void. But at its root is the word "voice." It means to call back. It's hard not to see a kind of familiar voicelessness in this canceling darkness. And what spells death to a poet better than being voiceless?
Even still, at the moment when darkness steals everything, right down to his very voice, the speaker loves his beloved. Aww.