This hymn is used for Sext on Sundays and Weekdays throughout the year in the Extraordinary form and the Ordinary Form.
Lord of eternal truth and might!
Ruler of nature’s changing scheme!
Who dost bring forth the morning light,
And temper noon’s effulgent beam:
Quench Thou in us the flames of strife,
And bid the heat of passion cease;
From perils guard our feeble life,
And keep our souls in perfect peace.
Father of mercies! hear our cry;
Hear us, O sole-begotten Son!
Who, with the Holy Ghost most high,
Reignest while endless ages run.
Words: Attr. St. Ambrose, 4th C.; tr. Fr. Edward Caswall, 1849.
Tune: “Rector Potens Verax Deus” Gregorian Chant, Mode II, traditional.*
*This tone is used for Sundays throughout the year. There are a number of other musical settings for this hymn.
The original Latin text may be found here.