If, in thy second state sublime, Thy ransom'd reason change replies With all the circle of the wise, The perfect flower of human time;
And if thou cast thine eyes below, How dimly character'd and slight, How dwarf'd a growth of cold and night, How blanch'd with darkness must I grow!
Yet turn thee to the doubtful shore, Where thy first form was made a man; I loved thee, Spirit, and love, nor can The soul of Shakespeare love thee more.
Tennyson imagines Arthur watching him from Heaven in his perfected state ("state sublime").
He imagines his close friend hob-nobbing with all the fancy intellectual elites who are with him up there. That's the whole "circle of the wise" that Tennyson regards as being the highest perfection of humanity.
If Arthur looks down on him, he'll seem pretty low compared to those other guys.
The images he uses here really drive this point home. He's "dimly character'd," "slight" (which means small and insubstantial), and like a dwarf.
But Tennyson's got something going for him that all these other wise souls—including Shakespeare—don't: his love for Arthur. That will never be equaled.