What's the exact meaning of "Did one but know" at the end of "Sonnet" by Christina Rossetti?
This is the full context：
by Christina Rosetti
I wish I could remember that first day,
First hour, first moment of your meeting me,
If bright or dim the season, it might be
Summer or winter for aught I can say;
So unrecorded did it slip away,
So blind was I to see and to foresee,
So dull to mark the budding of my tree
That would not blossom yet for many a May.
If only I could recollect it, such
A day of days! I let it come and go
As traceless as a thaw of bygone snow;
It seemed to mean so little, meant so much;
If only now I could recall that touch,
First touch of hand in hand – Did one but know!
And can anyone explain the meaning of 'but' here?