Which limb lies
Next the mirror?
For neither is clearer
Nor a different colour
Than the other,
Nor meets a stranger
In this arrangement
Of leg and leg and
Arm and so on.
To his mind
It's the indication
Of a mirrored reflection
Somewhere along the line
Of what we call the spine.
He felt in modesty
His person was
Half looking-glass,
The mirror quite literally splits him in half. 
 The image is just like the game children play where they pretend that they are floating by standing half on one side of a mirror, half on the other.
The Gentleman of Shalott