I'm having trouble finding this hymn in the usual books (I do see a thread on Christe qui splendor), or indeed the Liber Vesperalis Mediolanensis. It is familiar to me from Lutheran sources as Christe qui lux es, but Palestrina sets that same text; the edition on CPDL appears to have the even verses out of order.
Have you tried the Analecta Hymnica? I am a few 100 miles away from my Library, so can not do a Proper search, but could do one late August when I am back.
EDIT. AH Vol 51 pg 112? it seems to be in more than one volume!
Coverage of the Office has much increased at Gregobase and I suppose I should have started there. There are 7 tunes from Liber Hymnarius (Solesmes, 1983) for Christe qui splendor (Urban's? revision of the hymn) and 2 more for Christe qui lux. This hasn't gotten me closer to Palestrina's source, though.
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