Jewish Mysticism
“And there in a cloud appeared the Presence of the Lord” (Exod. 16-10), in order that the glory that is within it would not be seen, as it says, “He made darkness His screen” (Ps. 18-12), “a cloud and thick darkness are around Him” (ibid. 97-2), i.e., surrounding the Holy One, blessed be He, there is a cloud and thick darkness. This is to say, the inner glory that is within it is not seen, even by his servants and messengers who are sent before Him, but the thick darkness itself is the transparent and translucent light, as the sages, blessed be their memory, said in the prayer, “[You appeared to them] in bright clouds,” and these are the sefirot. Their light in relation to His light is like the light of the candle in relation to the sun, and they are like a clear glass that shines and illuminates, showing to the eye what is within it. The tenth sefirah is the speculum that does not shine, and it is like a glass mirror, and the one who looks at it sees His image within it, and that which is within it is not seen outside it… Therefore this sefirah is called the cloud of glory, for the glory is concealed in it. The rabbis, blessed be their memory, called it the speculum that does not shine because of the glory that is hidden within it. When the Holy One, blessed be He, wills to talk to His prophets, this sefirah becomes filled from the inner glory, according to their level, and they hear the word, but the one who speaks is not seen by them for He is hidden within it.

Translated by Elliot Wolfson in Through a Speculum that Shines- Vision and Imagination in Medieval Jewish Mysticism, Princeton- Princeton University Press, 1994.