Ruskin is referring to Spenser 's The Faerie Queene. The original text reads:
But set the truth and set the right aside,
For they with wrong or falshood will not fare;
And put two wrongs together to be tride,
Or else two falses, of each equall share;
and then together doe them compare.
For truth is one, and right is euer one.
So did he, and then plaine it did appeare,
Whether of them the great were attone.
But right sate in the middest of the beame alone.
( Spenser, The Faerie Queene, Book 5, Canto 2, Stanza 48)