Joseph Pieper, Leisure: the basis of culture, trans. Alexander Dru (San Fransisco: Ignatius Press, 2009 [1963/1952]) , 73-74. The Latin is from the Preface of the Nativity (Octave of Christmas, i.e. Christmas Day through Epiphany Eve, with the exception of the Feast of St. John), and goes back to the Gregorian Sacramentary (nos. 38, 51, and 1537) at least. See The Gregorian sacramentary under Charles the Great: edited from three mss. of the 9th century by H. A. Wilson, Henry Bradshaw Society 49 (London: Henry Bradshaw Society, 1915), :
for through the mystery of the Word made flesh thy splendour has shone before our mind's eye with a new radiance, and through him whom we recognize as God made visible we are carried away in love of things invisible(The MIssal in Latin and English, being the text of the Missale Romanum with English rubrics and a new translation (New York: Sheed & Ward, 1959), 763).