See Heb. i. 3. “Splendor” is St. Ambrose’s rendering of ἀπαύγασμα. Theodoret says: “The radiance” (or “effulgence”) “of a fire comes from it and accompanies it. The fire causes the radiance, but the radiance is inseparable from the fire. Also the radiance of the fire is of the same nature with it; so also is the Son of the same nature with the Father.” Theophylact—“The sun is never seen without his radiance, and we cannot think of a father without his child.” Delitzsch—“It is no nimbus around God that is here called His “glory,” but God’s own inconceivable, spiritual fire and brightness (die übersinnliche geistige Feuer und Lichtnatur Gottes selber), which He, in order to reveal Himself to Himself, makes an object to Himself” (aus sich heraussetzt).