Several weeks ago, a participant in an online discussion group for choral enthusiasts, who reported that he was setting a poem by Emily Dickinson (#1083), expressed puzzlement over this phrase in the opening lines:
We learn in the retreating
How vast an one
Was recently among us.
He was puzzled by what he called the “very strange” sound of “an one" and wanted to know if he could “get away with” changing it to “a one” in a choral setting that he was composing.
I would leave Dickinson’s poem as it is, for three reasons.
First, the use of "an" before words that start with "W" or "H" ("an one," "an historical event") was not unusual in Dickinson's time. The suggested change (“a one”) might be more in line with our modern practice, but it dilutes the color of her language, the formality of her approach, and the sense of her time and place.
Second, to use the poem at all, and to make any changes to it, one would need to obtain permission from the party or parties that hold copyright to Dickinson's works.
Third – and I am speaking as a poet here – it never seems a good idea to change a poet’s words unless there is a truly compelling reason to do so. She is dead and cannot defend herself, but if she could, she might point out the alliteration between "one" and "was," as well as the fact that the use of "an" instead of "a" requires the speaker or singer to distinguish the word "one." That is, the natural elision in "a one" renders the word "one" less distinctive to the ear. She's putting forth a big idea – "how vast an one" – and she uses the more formal, more distinctive sound of "an" to bring our ear to "one." Try speaking the lines aloud, first with "a one" and then with "an one." She knows what she's doing.
Consider the whole poem – its structure, its feel in the mouth, its sound to the ear, and of course its messages – not just this one spot that might confound our modern ideas.
A better choice for this composer might be to set Dickinson's words as she wrote them, then be sure that all the texts are printed in the concert program. Program notes can address any textual questions, including clarifying any now-archaic usages that might be unfamiliar to the audience. But I'm sure that if an audience reads "how vast an one" in the printed program, they'll understand it. (I am always in favor of printing all texts in vocal programs, even for English-language selections.)
Last summer I sang some new choral settings of several Dickinson poems. The composer had, in two or three instances (albeit with permission), changed Dickinson's texts. In one case, a different title was given to a poem (confounding both director and program annotator!); in another, an old word that sounds offensive but isn't ("niggardly," which means "miserly" and has nothing to do with race) was replaced with a different, softer-sounding word that just didn't mean the same thing at all. This last-named change destroyed a really wonderful internal rhyme and removed much of the consonant-generated forcefulness of the phrase. These changes did not improve the poems, and to be honest, it made me wonder about the composer's sympathies and understanding.