And if half his head's reflected,
Thought, he thinks, might be affected.
But he's resigned
To such economical design.
If the glass slips
He's in a fix –
Only one leg, etc. But
While it stays put
He can walk and run
And his hands can clasp one
Another. The uncertainty
He says he
Finds exhilarating. He loves
That sense of constant re-adjustment.
He wishes to be quoted as saying at present:
‘Half is enough.'
Perhaps Bishop is explaining her own life, her own way of coping with extreme circumstances. Half-experiencing life is enough, she seems to say, as it seems to work at present. 
 (Source:  Open Yale Courses ) 
 The gentleman’s satisfaction places him in contrast with Tennyson’s Lady of Shalott, who is  tired of experiencing life as shadows in the mirror: 
 “I am half-sick of shadows,” said The Lady of Shalott.
The Gentleman of Shalott