W.B. Yeats has created rhythm in his poem "When You Are Old" by using a familiar meter, simple rhyme scheme and by enhancing these forms with effective poetic devices and substitutions.
Almost everyone who has studied English has read a play written by William Shakespeare. Yeats uses the same form, iambic pentameter, to create a steady rhythm that is familiar to many readers. He uses substitution feet to deviate from the regular meter and emphasize the parts of the poem he feels are important. For example, he uses a spondaic foot to make the alliteration "glad grace" stand out. To maintain his steady rhythm he also uses phyrric feet to rapidly flow through small words and prepositions. The second part of the line, "...and of their shadows deep;" uses one of these feet to keep 'of' from being stressed, adding to the effect of the imagery of the eyes in the past.
The use of a simplistic rhyme scheme does not mean the poem is simple by any means. In fact, it is just the opposite. The use of an ABBA CDDC EFFE rhyme scheme is a strategic decision by the author to help create constant rhythm and repetition. Each stanza is one long poetic sentence that is held together with rhyme. The rhyming couplets in the 2nd and 3rd lines make the rhythm flow. The last word of the 4th line enhances the lyricism, completing the thought by connecting the 1st line with rhyme.
Along with his expert knowledge of poetic form Yeats uses a wide range of poetic devices to create rhythm. In this line he uses a substitution foot with alliteration to enhance his theme and make the happy memories more memorable to the reader: "How many loved your moments of glad grace." The themes of romance and loss are important and are elaborated on with the personification of Love: "Murmur, a little sadly, how Love fled and paced upon the mountains overhead and hid his face amid a crowd of stars." These devices are used appropriately because they stay within the steady rhythm that has already been created, adding to its overall effectiveness.
In summarization, W.B. Yeats experimented with many forms of poetry in his career and has a deep understanding for the presentation of the subject matter. He demonstrates this by creating a perpetual rhythm that when varied can be analytically recognized. He uses the familiar iambic pentameter; ABBA rhyme scheme; and wide range of literary devices to present his poem with a stable rhythm and smooth flow.
Yeats, William Butler. "When You're Old," 1865.