"Nature" is what we see
The Hill—the Afternoon
Squirrel—Eclipse—the Bumble bee
Nay—Nature is Heaven
Nature is what we hear
The Bobolink—the Sea
Thunder—the Cricket
Nay—Nature is Harmony
Dickinson moves on to an aural definition of Nature.  Note this echoes the opening line and is echoed by the closing line.  This is an example of  anaphora  and also  syntactic parallelism .
Nature is what we see—