For they walked not with their eyes shut, but there was something within them which did not permit them to know that which they saw, which a mist, darkness, or some kind of moisture, frequently occasions. Not that the Lord was not able to transform His flesh that it should be really a different form from that which they were accustomed to behold; since in truth also before His passion, He was transfigured in the mount, so that His face was bright as the sun. But it was not so now. For we do not unfitly take this obstacle in the sight to have been caused by Satan, that Jesus might not be known. But still it was so permitted by Christ up to the sacrament of the bread, that by partaking of the unity of His body, the obstacle of the enemy might be understood to be removed, so that Christ might be known.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Con. Ev. lib. iii. c. 25.