On one occasion when the vested Priest went up to the Altar I saw a brilliant light coming down from Heaven, illuminating the whole Altar. This light was not withdrawn until the Priest left the Sanctuary at the conclusion of Mass. I noticed that when the Priest got to the Sanctus and began the Canon, a flame of extraordinary brightness shot down from above upon the bread and wine, illuminating them with its light as the rays of the sun makes glass shine.

Upon this stream of light the sacred elements rose to Heaven and when they descended they were transformed into true Flesh and Blood, though to the eye of man they still appeared to be bread and wine. As I gazed upon this Flesh and Blood I saw the signs of the Incarnation, the Birth, the Passion of Our Savior reflected in them as in a mirror, and just as we know these events to heave been accomplished when the Son of God was on earth."

This vision of St. Hildegarde shows us in how wondrous a manner the transubstantiation of the bread and wine is effected by the penetrating power of the Divine Fire as, by the power of natural fire, wood is transformed into glowing embers. That which this holy Abbess was privileged to behold occurs in every Mass: a celestial radiance surrounds the Altar from the beginning to the end of Mass, and at the time of Consecration a flame of dazzling brightness descends from Heaven and changes the substances of bread and wine into the true Body and Blood of Christ.

The Mass Of St. Clement, Bishop and Martyr

It is related of the holy Bishop and Martyr, Clement, that being sent by Diocletian to the island of Rhodes, where at the request of the Bishop, he celebrated Mass and at the time of the Consecration the Sacred Host was changed into the likeness of a live coal in the sight of all present. A multitude of Angels were also seen hovering around the Host in joyful and happy adoration. The miraculous Coal glowed with a brilliance so dazzling that the congregation could not bear to look at it and fell upon their faces, remaining in this posture till after the Communion. Not until Clement prepared, with great reverence, to consume the Sacred Host did it resume Its former appearance.

Source: Mary Help of Christians.