The greater part of his life, however, was devoted to the collection, classification, and interpretation of the countless treasures of medieval art surviving in France, Belgium, Germany, and elsewhere in Europe.
As early as 1840 he began his collaboration with his Jesuit confrére, Arthur Martin, an excellent draughtsman, and chief collector of the mass of artistic material that Cahier classified and interpreted.
(Paris, 1848–59), four quarto volumes of illustrated dissertations on gold and silver church-plate, enamelled ware, carved ivories, tapestries, bas-reliefs, and paintings belonging to the Carolingian and Romanesque periods (9th to 12th century).
In spite of his numerous digressions and parentheses, says Joseph Brucker,[2] and a somewhat neglected style, Cahier is never wearisome; a vein of kindly but caustic humor runs through his pages, in which about pungent words and phrases, dictated, however, by candour and the love of truth.
He was deeply versed in all kinds of curious medieval lore, and particularly in the "people's calendar" or every-day usages and customs connected with the liturgical life of the Catholic church.