Eleazar of Worms, Sode Razayya, p. 8

Jewish Mysticism
The angels who are sent below [appear] according to the need of the hour and according to the will [of God]. Elijah made himself appear like a bear in order to bother R. Hiyya and his son [as is related in the Babylonian talmud, tractate] Bava Mesi‘a [85b]… At that moment Elijah was not a bear but he appeared to them as such. Similarly an angel came to Jacob in the image of Esau … The angels are very subtle bodies, like the spirit which is subtle and not visible, for the body [of the angel] is from the spirit and from fire. If He desires to reveal them He makes their bodies [literally, their burden] heavier, [as it says] “Then one of the seraphs flew over to me” (Isa. 6-6), and in Daniel, “[I looked and saw] a man dressed in linen” (Dan. 10-5). For the Holy One, blessed be He, strengthened the light of the eye, as it says, “And the Lord opened the servant’s eyes” (2 Kings 6-17). The Holy One, blessed be He, allows the angel to be seen well and his body will be coarse and not subtle, and there is a conflict between him and those people who stand there and do not see him… The angel is replete with images (da‘atanot) and the [particular] perspective of a person is according to the decree [that has been established].

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