The likeness of the firmament. — The word rendered “firmament” has undoubtedly originated, etymologically, from a verb originally signifying to beat out, as in the case of metals; but the derivative word, in its use in connection with the heavens, had wholly lost this reference, and had come to mean simply an expanse. The Hebrews do not appear to have ever entertained the classical idea of the sky as a metallic vault, the only passage seeming to indicate such a notion (Job 37:18) being capable of quite a different explanation. We are here to conceive, therefore, of that which was “stretched forth over their heads above” as a simple expanse, like the sky, as if he had said, “And above their heads was stretched forth the sky.” This expanse is not represented as supported by the cherubim, or resting upon them, and it remained undisturbed when they let down their wings (Ezekiel 1:25). It was simply “stretched forth over their heads,” at once separating them from, and yet uniting them with, the throne above. It fulfils, therefore, the complementary part to the wheels. They connected the vision with the earth; this connects it with God.
The colour of the terrible crystal — The expression “crystal” is doubtless derived from Exodus 24:10, as in turn it became the foundation for Revelation 4:6. Yet it is not here any particular crystal; the word is Merely used to convey some idea of the appearance of the expanse beneath the throne, clear as crystal, terrible in its dazzling brightness.