Ostensorium means, in accordance with its etymology, a vessel designed for the more convenient exhibition of some object of piety. Both the name ostensorium and the kindred word monstrance (monstrancia, from monstrare) were originally applied to all kinds of vessels of goldsmith's or silversmith's work in which glass, crystal, etc. were so employed as to allow the contents to be readily distinguished, whether the object thus honoured were the Sacred Host itself or only the relic of some saint. Modern usage, at any rate so far as the English language is concerned, has limited both terms to vessels intended for the exposition of the Blessed Sacrament, and it is in this sense only that we use ostensorium here.
It is plain that the introduction of ostensoria must have been posterior to the period at which the practice of exposing the Blessed Sacrament or carrying it in procession first became familiar in the Church. This (as may be seen from the articles BENEDICTION OF THE BLESSED SACRAMENT, CORPUS CHRISTI, and EXPOSITION OF THE BLESSED SACRAMENT) cannot be assigned to an earlier date than the thirteenth century. At the same time, Lanfranc's constitutions for the monks of Christ Church, Canterbury (c. 1070), direct that in the Palm Sunday procession two priests vested in albs should carry a portable shrine (feretrum) "in which also the Body of the Lord ought to be deposited". Although there is here no suggestion that the Host should be exposed to view but rather the contrary, still we find that this English custom led, in at least one instance, to the construction of an elaborately decorated shrine for the carrying of the Blessed Sacrament on this special occasion. Simon, Abbot of St. Albans (1166-83), presented to the abbey a costly ark-shaped vessel adorned with enamels representing scenes of the Passion, which was to be used on Palm Sunday "that the faithful might see with what honour the most holy Body of Christ should be treated which at this season offered itself to be scourged, crucified and buried" ("Gesta Abbatum", Rolls Series, I, 191-92). That this, however, was in any proper sense an ostensorium in which the Host was exposed to view is not stated and cannot be assumed. At the same time it is highly probable that such ostensoria in the strict sense began to be constructed in the thirteenth century, and there are some vessels still in existence — for example, an octagonal monstrance at Bari, bearing: the words "Hic Corpus Domini" — which may very well belong to that date.
Reference:
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