And the likeness of the firmament upon the heads of the living creature,.... This was not a real firmament, but the likeness of one; it was like the firmament which God created on the second day, which divided between the waters above and below, and which he called Heaven; and is no other than the airy and starry heavens.
Its name is in Hebreweyqr, "an expanse"; from its being stretched out as a curtain, and a tent to dwell in; and a "firmament" from its firmness and continuance; and therefore called the firmament of his power, Ge 1:8 Ps 150:1; and this visionary one was "over the heads of the living creatures"; which shows that they could not be angels, for those have their habitation in the third heaven, above the firmament; much less in hieroglyphic of the trinity of Persons, who are the three that bear record in heaven, and are not under the firmament; but ministers of the Gospel, who are on earth, and are subject to Christ, whose throne is above the firmament, Eze 1:26; and who receive their commission and gifts from him, and are accountable to him.
This firmament[was] as the colony of the tenable crystal; crystal is a very white, transparent, precious stone, resembling ice, from whence it has its name; hence Pliny thought it was no other than ice vehemently frozen; and here it is called "terrible", because exceeding clear and bright, so that there was no looking upon it, without the eyes being dazzled with the glory of it.