
In a line that's supposed to have eight syllables, we get nine. Then, the line ends with a single stressed syllable-snow. The first two groups are iambs (daDUM daDUM), but the next two contain a stressed syllable followed by an unstressed syllable-DAdum. You'll notice that there are an odd number of syllables here, which means our line isn't perfect, per se. Unfortunately, Blake was never one to keep things neat and pretty. That means it consists of four iambs ("and smiled" is one, "among" is one, and so forth), all in a neat little row. That, ladies and gentlemen, is what we call a perfect line of iambic tetrameter.
