For the complete book, see Google Books
https://books.google.com/books?id=e30rAAAAYAAJ&dq=bushnell+%22Nature+and+the+Supernatural%22&source=gbs_navlinks_s
or see University of Michigan
http://quod.lib.umich.edu/m/moa/agv8944.0001.001/13?page=root;rgn=full+text;size=100;view=image

Bushnell, Horace. Nature and The Supernatural, Charles Schribner, New York, 1859.

This one page by Horace Bushnell, published in 1859,  presents a good background for understanding La Farge’s comments about how Winslow Homer viewed Nature.