The Butterfly as a Mirror 

"People have thought she tried to cross the lake at Lochan Neck where zesty skaters crossed from Exe to Wye on days of special frost. Others supposed she might have lost her way by turning left from Bridgeroad; and some say she took her poor young life. I know. You know."

As Mary McCarthy said in her essay "Love is the burden of Pale Fire, love and loss. Love is felt as a kind of homesickness, that yearning for union described by Plato, the pining for the other half of a once-whole body,"
Hazel shade looked at her reflection "Lady in the Lake" in the icy lake Omega. It was her yearning to cross the others side and unite the body that led her to cross the icy layer.
If we think of the butterfly as being split in half it resembles itself. The wings split by the thorax is like a the ice splitting the air and water or a window splitting the outside from inside.
This moment splits the poem and is where Shade goes from imagining the other wing to looking at the reflection of the ice, water, or hazel window pane. It is a reflection but all reflection have some difference or mole (Lines 737 - 766 say it all). At first glance at the butterfly above the two wings look the same but with a closer look we can see slight differences in the shades of colors especially closer to the outside of the wings. There has been no prof of crossing "The Land Beyond the Veil" but we can have a better explanation of the others side by paying attention to the details of the reflection we are looking at.
As the book as a whole one wing is the unfinished poem and the other is the commentary to the poem. It is our job to look at the details in both sides to find what the reflection is that has been seen by kinbote and Shade. Its working backwards but as is the life of the reader. Details are the imperfections of the reflection.