(From ostendere, "to show").
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.
A large number of medieval ostensoria have been figured by Cahier and Martin (Mélanges Archéologiques, I and VII) and by other authorities, and though it is often difficult to distinguish between simple reliquaries and vessels intended for the exposition of the Blessed Sacrament, a certain line of development may be traced in the evolution of these latter. Father Cahier suggests with some probability (Mélanges, VII, 271) that while at first the ciborium itself was employed for carrying the Blessed Sacrament in processions, etc., the sides of the cup of the ciborium were at first prolonged by a cylinder of crystal or glass, and the ordinary cover superimposed. Such a vessel might have served for either purpose, viz., either for giving Communion or for carrying the Host visibly in procession. Soon, however, the practice of exposition became sufficiently common to seem to require an ostensorium for that express object, and for this the upright cylindrical vessel of crystal was at first retained, often with supports of an architectural character and with tabernacle work, niches, and statues. In the central cylinder a large Host was placed, being kept upright by being held in a lunette (q. v.) constructed for the purpose. Many medieval monstrances of this type are still in existence. Soon, however, it became clear that the ostensorium could be better adapted to the object of drawing all eyes to the Sacred Host itself by making the transparent portion of the vessel just of the size required, and surrounded, like the sun, with rays. Monstrances of this shape, dating from the fifteenth century, are also not uncommon, and for several hundred years past this has been by far the commonest form in practical use.
Of course the adoption of ostensoria for processions of the Blessed Sacrament was a gradual process, and, if we may trust the miniatures found in the liturgical books of the Middle Ages, the Sacred Host was often carried on such occasions in a closed ciborium. An early example of a special vessel constructed for this purpose is a gift made by Archbishop Robert Courtney, an Englishman by birth, who died in 1324, to his cathedral church of Reims. He bequeathed with other ornaments "a golden cross set with precious stones and having a crystal in the middle, in which is placed the Body of Christ, and is carried in procession upon the feast of the most holy Sacrament." In a curious instance mentioned by Bergner (Handbuchd. Kirch. Kunstaltert mer in Deutschland, 356) a casket constructed in 1205 at Augsburg, to hold a miraculous Host from which blood had trickled, had an aperture bored in it more than a century later to allow the Host to be seen. Very probably a similar plan was sometimes adopted with vessels which are more strictly Eucharistic. Early medieval inventories often allow us to form an idea, of the rapid extension of the use of monstrances. In the inventories of the thirteenth century they are seldom or never mentioned, but in the fifteenth century they have become a feature in all larger churches. Thus at St. Paul's, London, in 1245 and 1298 we find no mention of anything like an ostensorium, but in 1402 we have record of the "cross of crystal to put the Body of Christ in and to carry it upon the feast of Corpus Christi and at Easter". At Durham we hear of "a goodly shrine ordained to be carried on Corpus Christi day in procession, and called 'Corpus Christi Shrine', all finely gilded, a goodly thing; to behold, and on the height of the said shrine was a four-square box all of crystal wherein was enclosed the Holy Sacrament of the Altar, and it was carried the same day with iiij priests" (Rites of Durham, c. lvi). But in the greater English churches a preference seems to have been shown connected no doubt with the ceremonial of the Easter sepulchre, for a form of monstrance which reproduced the figure of Our Lord, the Sacred Host being inserted behind a crystal door in the breast. This, at any rate, was case, i.e. in the Lincoln, Salisbury, and other famous cathedrals. These statues, however, for the exposition of the Blessed Eucharist seem to have been of comparatively late date. On the continent, and more particularly in Spain, a fashion seems to have been introduced in the sixteenth century of constructing ostensoria of enormous size, standing six, seven, or even, feet in height, and weighing many hundreds pounds. Of course it was necessary that in such cases the shrine in which the Blessed Sacrament was more immediately contained should be detachable, so that it could be used for giving benediction. The great monstrance of the cathedral of Toledo, which is more than twelve feet high, and the construction of which occupied in all more than 100 years, is adorned with 260 statuettes, one of the largest of which is said to be made of the gold brought by Columbus from the New World.
In the language of the older liturgical manuals ostensorium is not infrequently called tabernaculum, and it is under that name that a special blessing is provided for it in the "Pontificale Romanum". Several other designations are also in use, of which the commonest in perhaps custodia, though this is also a specially applied to the sort of transparent pyx in which Sacred Host is immediately secured. In Scotland, before the reformation, an ostensorium was commonly called a "eucharist", in England a "monstre or "monstral". The orb and rays of a monstrance should at least be of silver or silver gilt, and it is recommended that it should be surmounted by a cross.
An excellent chapter in CORBLET, Histoire du Sacrement de l'Eucharistie, II (Paris, 1882), gives a general account with a description of many famous ostensoria. SCHROD in Kirchenlexikon, s.v. Monstranz; RAIBLE, Der Tabenakel einst und jetzt (Freiburg, 1908); THURSTON, Benediction of the Blessed Sacrament in The Month (July, 1901); OTTE, Handbuch der kirchlichen Kunst-Arch ologie, I, (Leipzig, 1883), 208-210; MARTIN AND CAHIER, Mélanges arch ologiques, I, VII (Paris, 1847-75); REUSSENS, Archéologie chrétienne, II, 334 sqq.; BARBER DE MONTAULT, Les ostensoires du XIVe siècle en Limousin in the Congrès Archéolog. de France, 1879, 555-590. See also articles too numerous to specify in the Revue de l'Art Chrétien and the Zeitschrift für christliche Kunst, where many excellent reproductions of medieval monstrances will be found.