Brass and iron as in Isaiah 48:4 and Daniel 7:19 signify what is hard.

In Genesis 4:22, brass signifies natural good. (Arcana Coelestia 421)

In Numbers 21:9, this signifies the Lord's divine natural. (Apocalypse Revealed 49)

Brass also signifies the good in man's natural and the sensuous in the Lord. (Arcana Coelestia 197, Apocalypse Revealed 49, Apocalypse Explained 705)

(Odkazy: Apocalypse Explained 70)

37. And every eye shall see Him, signifies that all will acknowledge Him who are in truths from good. This is evident from the signification of "seeing," as being to understand, to perceive, and to acknowledge (See Arcana Coelestia 2150, 2325, 2807, 3764, 3863, 3869, 4723, 10705); and from the signification of "eye," as being understanding and faith (See Arcana Coelestia 2701, 4403-4421, 4523-4534, 9051, 10569); thus such as are in truths from good, for these are in faith and thence in understanding. That the "eye" signifies understanding and faith, is from correspondence; for understanding is internal sight, and it is internal sight that sees by means of the eye, for that sight determines the eye to objects and disposes its interior parts, which are many, for reception. It is in consequence of this that the iris and pupil seem now dilated, now contracted, now obscure, now bright and sometimes sparkling, altogether in agreement with the desire and want of the understanding. Therefore, to some extent, the affection of the thought can be seen from the eyes. It is from this correspondence that the eye signifies the understanding. The eye also signifies faith, because truths of faith have no other abode than the understanding; truths that do not abide there are not in the man (See Doctrine of the New Jerusalem,n. 28-36). The "eye" is, in the spiritual sense, faith, also because of the angels looking to the Lord; for angels turn their faces continually to the Lord, and see Him by faith and understanding. (On which turning, see the work on Heaven and Hell 123, 142, 143, 145, 151, 153, 255, 272, 510)