William Wordsworth's short lyric "My Heart Leaps Up When I Behold" is part of a larger collection titled Poems, in Two Volumes (1807. As with Wordsworth's other lyrics, the poem exemplifies the nature-centered view of the Romantic Period, which blossomed in Britain beginning in the 1790s and a reaction against the intellectual, coldly rational poetry of the Augustan Age in the mid-to-late 18thC. For Wordsworth and poets like Coleridge, Shelley, Keats, and Byron, an understanding of and appreciation for nature became the path to man's ultimate salvation.
With some variations, the predominate form of the poem is iambic tetrameter, that is, four sets of unstressed/stressed syllables or four beats in a line. Those who study sound patterns in English have concluded that the most natural rhythm for English is iambic (one set of unstressed/stressed syllables) pentameter, five sets of iambs. In this case, however, Wordsworth has chosen to use four sets of iambs, so we have iambic tetrameter. In the first line of the poem--A rainbow in the sky--we have iambic trimeter, three sets of iambs rather than four (tetrameter). If you read the poem out loud, you will note that a shift in the predominate pattern usually catches the reader's attention, so many poets, like Wordsworth, vary the rhythm in order to stop the reader from developing a sing-song rhythm that might dull the reader's understanding.
For Wordsworth and many Romantic poets, children are more perceptive and open to nature than adults, and here we have the best possible combination of innocence:
My heart leaps up when I behold/A rainbow in the sky:/So it was when my life began,/So is it now I am a man,/So be it when I shall grow old. . . .
The speaker, presumably a man in his middle age, looks back to the joy he felt as a child looking at a rainbow. The rainbow, perhaps one of the most joyous images of childhood and innocence, continues to provide joy to the middle-age man. More important, perhaps, the joy stretches through the years so that the man fully expects the rainbow to impart the same joy when he reaches old age. This joy is of such profound importance to the speaker that he would literally prefer to die without it.
The speaker indicates the importance of childhood and nature in a classic paradox:
The Child is father of the Man:/And I could wish my days to be/Bound each to each by natural piety.
The paradox--that a child could be father to a man--catches the reader off guard but imparts a not-so-obvious truth: every man grows out of his childhood and, in a sense, the child fathers the man. Many readers and critics have, at this point, noted a common saying among the Jesuits--"Give me a boy until he is seven, and I will give you the man--that implies a man is the result of his childhood.
For Wordsworth, however, the important point is that the man and boy are bound by their faith in the beauty of nature, symbolized by the rainbow that links their youth to middle age to old age. When Wordsworth uses the phrase "natural piety," he means that nature is at the center of faith, a faith that is memorialized by the rainbow as a constant presence in the boy and man.