Züccâce (glass): Regarding the imagination of the spirit (hayāl al-rūḥ), we can identify three characteristics. First, it possesses the structure of the dense and lowly world, because what is imagined has quantity, form, and limited aspects. It also has relationships such as remoteness and proximity. The density that is the attribute of bodies is characterized by veiling the purely intellectual lights from qualities such as direction, quantity, remoteness, and proximity.
The second is this: When this dense imagination is purified, refined, adorned, and controlled, it can become commensurate with intellectual meanings and can convey intellectual lights without preventing their reflection.
The third is this: In the beginning, imagination needs the intellect in order to retain intellectual maʿrifa (gnoses). It does not become disturbed, shaken, or distracted by other things when it remains within the bounds of retention. In this way, these imaginative examples gather together intellectual knowledge. These three characteristics, in relation to the lights that cause vision, cannot be found in any other substance in the world of shahāda (the world of witnessing) except glass. For glass is an original substance, it is dense, but it has been purified and refined to such an extent that no veil remains before the light of the lamp. Thus, it protects the lamp from being shaken and extinguished by strong winds. (Mishkāt, 84)
In the verse of Nūr (light), it is alluded to. That is the qalb (the heart). The miṣbāḥ (lamp) is the rūḥ (the spirit). The burning tree is from it. The züccâce, which resembles shining stars, is the nafs (the lower self). The mishkāt (niche) is the body. (Kāshānī, 55; Jāmiʿ, 85)
(See also: nūr-zulmat)