The tone of the end of the poem is curious and disappointed, and the mood is regretful and thoughtful.

Tone is the author’s attitude toward the subject, and the mood is the emotional landscape of the poem.  In this poem, Frost uses a well as a metaphor for life.  He looks at the well not to see what is at the bottom, but to see himself. He needs to see himself first.  He waits until the well, “Gives me back in a shining surface picture/Me myself in the summer heaven godlike.”  In order to understand the world, and life, he has to see where he is himself first.

At the end of the poem he loses what he started to see.  He wonders what it was, a little sadly and a little curiously.

One drop fell from a fern, and lo, a ripple
Shook whatever it was lay there at bottom,
Blurred it, blotted it out. What was that whiteness?
Truth? A pebble of quartz? For once, then, something.

He will never know what it was he saw in that one glimpse, but he still saw more than others see because he was looking.  He was looking in a different way than his friends looked, and he was seeing things they could not see because he saw things when looking at himself.

The tone, professed in words like “whiteness” to refer to the unknown, “blurred” and “blotted out” to refer to understanding, creates a mood that is not entirely sorrowful but still wistful.  Something might have been, and understanding might have been reached, but at least he came close and saw a glimpse.