Sarum Kyrie eleison. No. 6.

VI. Lux et Origo
On Holy Cross Day especially.

O Light, source of the highest light, God over all, have mercy on us. 

All things came to be at your command, have mercy on us.

You alone can have mercy, have mercy on us.

Redeemer of humanity and our salvation, have mercy on us.

By your cross you redeemed us from everlasting death, have mercy on us.

You are the Word of the Father, the author of mercy, and the Light of truth, have mercy on us.

O Counselor, Holy Spirit, God, have mercy on us.

You are our compassionate physician, have mercy on us.

O Holy Trinity and unity, always, have mercy on us.

Source: The Sarum Missal in English, Part II, Alcuin Club Collections, No. XI

A variant text has this in place of the last three petitions above:

O Adonai, Lord God, just Judge, have mercy.
Who governest the engine of things, O nurturing Father, have mercy.
Whom alone becometh praise and honour, now and always, have mercy.

Original in Latin:

Lux et origo lucis summę deus eleyson.
In cuius nutu constant cuncta clemens eleyson.
Qui solus potes misereri nobis eleyson.
O mundi redemptor cunctorum et factor nostri christe eleyson.
Per crucem redemptis a morte perhenni rex pie christe eleyson.
Qui es uerbum patris salus et humana lux uera christe eleyson.
Adonay domine deus iuste iudex eleyson.
Qui machinam gubernas rerum alme pater eleyson.
Quem solum laus et honor decet nunc et semper eleyson.

Original in Latin, with Gregorian notation: