In the poem, the phrase "sill of shade" refers
to a part of the scenery that is left unnoticed by many. The narrator of the
poem is trying to describe the details of the beauty of nature. He is in awe of
the surroundings that he has encountered.
The main motif of the poem is that Dickinson compares poetry, obviously and famously her favorite form, to a house or building that extends ad infinitum, or forever--the possibilities of poetry are endless. I hope my answer has come to your help. God bless and have a nice day ahead!
The author choose to concede the point and offers no rebuttal