"Image of Light, Adieu --"

Image of Light, Adieu --
Thanks for the interview --
So long -- so short --
Preceptor of the whole --
Coeval Cardinal --
Impart -- Depart --

F (1586), 1882

This brief but incredibly clever poem functions to fit at least two distinct interpretations, both of which serve as recurring themes for Dickinson's poetry. The first of these readings views it as a commentary on faith and the transient nature of Dickinson's experiences with it, a frequent point of concern for her, as seen in similar poems such as "I've known a Heaven, like a Tent --."  Much like how the faith in that poem is said to "Pluck up its stakes, and disappear -- /Without the sound of Boards/Or Rip of Nail..." it is here seen to appear and disappear, "Impart -- Depart --" in much the same way, with the holy "Image of Light" providing only a short "interview" before going away and leaving Dickinson once again. 

The Image of Light bids adieu

The second possibility sees the poem as another in a long line of Dickinson's poems chronicling sunsets ("The Sun kept stooping -- stooping -- low!," "An ignorance a Sunset," and "Bring me the sunset in a cup" for some prominent examples). In this sense, the literal "Image of Light" provides its "Interview" in the form of a day. Dickinson also seems to be punning on "So long" as both a phrase for goodbyes (whose American usage Merriam-Webster interestingly traces back to none other than fellow poet, Walt Whitman) as well as a note on the relative simultaneous length and brevity of a day. 

The more complex fourth and fifth lines also manage to fit both of these readings in interesting ways, as the moniker "Preceptor of the whole" can easily work to describe both God in his relation to humanity and existence, and also the sun in its relation to the scope of a day cycle. The usage of the words "Coeval Cardinal" continue to further this dual interpretation. The Dickinson lexicon notes "Coeval" can mean both "contemporary" and "ancient," characteristics that can be attributed to God, who may be said to become more "contemporary" in those moments of faith in which the "Image of Light" appears before retreating into back the "ancient" and abstract when it leaves. Likewise, the Sun exists with us today while also being "pre-mortal." The lexicon also recognizes the religious connotation of "Cardinal" while noting its potential figurative representation of "horizon points at sunrise or sunset." Knowing Dickinson's sense of humor and wit, the idea that she meant for both of these interpretations to be plausible is certainly likely. 

https://www.merriam-webster.com/words-at-play/so-long-origin

http://edl.byu.edu/lexicon/term/602061

http://edl.byu.edu/lexicon/term/601748