What does Crane mean when he writes, "It is all hung by an invisible white hair. It trembles as birch limbs webbing the air." Please explain....

"My Grandmother's Love Letters," by Hart Crane, speaks to
the past and the memories the speaker has of his
grandmother.

She is gone, but memories, as well as mementos
of her life, remain for the speaker. The night offers no visible stars (as it's
raining), "but those of memory," and the speaker believes that there is plenty of room
for memory as there is room for the soft rain that
falls.

There is room enough, too, for his grandmother's
love letters, so long stored away that they have become fragile—"brown and soft" enough
to "melt as snow." There is great space for the speaker to recall his grandmother—and
wonder about "Elizabeth," the person— apart from the one he loved—so much more than a
mere memory. However, the steps into this "space" must be "gentle," especially if he is
tempted to delve into unknown territory. He takes the time to consider the letters that
will teach him more about this woman. (It is not hard to imagine
someone wanting to learn about a loved one who is gone, especially in that when we are
younger, we often don't know enough to ask questions while they are still with
us.)

The lines you have mentioned are rhyming couplets.
Each stands alone on a separate line—ended by a period rather than starting on the first
line and continuing to the second. One source suggests that the speaker is having second
thoughts, and that the periods are evidence that he is pausing. Does he really want to
search out the secrets not of his grandmother, but of "Elizabeth,"
a woman he really never knew?

In these
two lines, the "invisible white hair" literally calls up the image of his grandmother:
but because it is invisible, the connection is one felt rather than seen. To know what
the "hair" refers to, we need context, so we look at "It is all hung…" "It" may then
refer to the decision the speaker has to make as to whether he is prepared to meet
"Elizabeth"—the woman she was before: before he was born;
before she was a grandmother; and, probably
before she was a mother. The person she was when she received these
letters has not existed for a very long time—life changes dramatically when we marry and
have children…and grandchildren. What will he make of this unknown person who he is
forever linked to? Perhaps, too, because he is a grandson first, and a curious adult
second, the child within him is unsure if he wants to know who she
used to be because he has loved her always as a grandmother. The
decision is an important one; it hangs by a fragile and "invisible white hair." The rest
of the couplet creates a simile, or a comparison: the hair trembles "like" the limbs of
a tree spread out through the air. The word "trembles" provides a sense of uncertainty,
or fear, or even anticipation—or perhaps all three, as the speaker
ponders what step next to take.

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It is all hung by an invisible white
hair.

It trembles as birch limbs webbing the
air.

To my mind, there is a
decision waiting to be made about whether the speaker wants to discover a new woman in
the person of his grandmother. He may actually see the move with anticipation—the birch
limbs "web" through the air, lifting their arms to loftier places—toward the sun: and
"higher planes" are often symbolic of increased knowledge. I do not believe his choice
is necessarily between good or bad knowledge, but a "before" and "after" thing—for once
he crosses into the world of "Elizabeth," he cannot go back: he will forever know of
"Elizabeth" before their lives were joined.