Ballad Meter
Emily Dickinson was no stranger to church. She grew up in a religious household, and even spent a year at Mount Holyoke Female Seminary. So, that means that she was no stranger to hymns, either. The rhythm of the church hymns worked their way into her brain and were spat right out again in poem form. And she wasn't the only poet to do this. Way back in the day, before paper (never mind iPads), poets used to deliver their poems as song, and the form they chose was the ballad.
The ballad is a perfect form if you want to belt out your poem in song because of its rhythm and rhyme. There are alternating lines of iambic tetrameter. What's that mean in English? Well, each odd-numbered line has eight syllables, divided up into four iambs per line). An iamb is a two-syllable pair which starts with an unstressed syllable and ends on a stressed one: daDUM. Four of those bad boys in one line? You've got some sweet iambic tetrameter ("tetra-" means four) on your hands:
Tell all the truth but tell it slant — (1)
Hear the iambs? You should get daDUM daDUM daDUM daDUM. And for the even-numbered lines you should hear just one less daDUM. That's because those have iambic trimeter ("tri-" meaning three), daDUM daDUM daDUM:
Success in Circuit lies (2)
So, Dickinson's borrowing this meter from church hymns (check out "Amazing Grace"), but what about the rhyme scheme? We get perfect end rhymes on the even-numbered lines ("lies" and "surprise," "kind" and "blind"). But the odd-numbered lines are just… well, odd. They have very slight slant rhymes: "slant" and "Delight" have the same hard T ending (that's called consonance in the poetry biz), while "eased" and "gradually" have the same long E sound (or assonance). This kind of near rhyme actually is a pretty neat way to emphasize the speaker's advice to tell the truth "slant." We don't just get full-on rhyming, in the same way we're not meant to just belt out the truth full on. Pretty neat, huh?