This hymn, sung traditionally (formerly required) at all Expositions of the Blessed Sacrament, is actually the last two verses of a longer hymn now sung traditionally on Good Friday: Pange Lingua (which I will offer later):

Tantum ergo Sacramentum
Veneremur cernui;
Et antiquum documentum
Novo cedat ritui;
Praestet fides supplementum
Sensuum defectui.
Genitori, Genitoque,
Laus et jubilatio,
Salus, honor, virtus quoque
Sit et benedictio;
Procedenti ab utroque
Compar sit laudatio. Amen.10

Translation:

Down in adoration falling,
Lo, the Sacred Host we hail.
Lo, o’er ancient forms departing,
Newer rites of grace prevail;
Faith for all defects supplying
Where the feeble senses fail..

To the everlasting Father,
And the Son who reigns on high,
With the Holy Ghost proceeding
Forth from Each eternally;
Be salvation, honor, blessing,
Might, and endless majesty. Amen.