That it assume thy body, I allow,
And fix itself in thy lip, eye, and brow.
Whilst thus to ballast Love I thought,
And so more steadily to have gone,
With wares which would sink admiration,
I saw I had love's pinnace overfraught ;
Thy every hair for Love to work upon
Is much too much ; some fitter must be sought ;
For, nor in nothing, nor in things
Extreme, and scattering bright, can love inhere ;
Then as an angel face and wings
Of air, not pure as it, yet pure doth wear,
So thy love may be my love's sphere ;
Just such disparity
As is 'twixt air's and angels' purity,
'Twixt women's love, and men's, will ever be.
Another example of these spiritual beings using something pure, yet less so than themselves, lies in the medieval belief that each of the concentric spheres which supposedly held the stars had a governing angel of its own. 
 In an increasingly convoluted metaphor, Donne is saying that this relationship is comparable to that of the speaker’s love for an ideal and her embodiment of love.  The latter thus remains pure and retains the quality he seeks, while providing a vessel or instrument for his governing spirit. 

Air and Angels