"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
Just like poetry, Nature is  indefinable . Note Dickinson’s  definition  of poetry, in which the words have meaning only in terms of the impact on the reader. 
 The negative “Nay” is an  archaism  which adds emphasis in the way “No” cannot. She ends the four lines by equating “Nature” and “Heaven”.  So though she expresses the concept in concrete visual terms, ultimately it is as indefinable as the idea of “Heaven”. 
 This line is echoed by line eight.
Nature is what we see—