The ostensorium (ostensory, monstrance) is a glass-framed shrine in which the Blessed Sacrament is publicly exposed. It may be of gold, silver, brass, or copper gilt (Cong. Sac. Rit., 31 August, 1867). The most appropriate form is that of the sun emitting its rays to all sides (Instructio Clement., 5). The base should be wide, and at a short distance above it there should be a knob for greater ease in handling. The ostensorium must be surmounted by a cross. (Cong. Sac. Rit., 11 September, 1847). It should not be embellished with small statues of saints, as these and the relics of saints are forbidden to be placed on the altar during solemn Benediction. At the sides of the receptacle in which the lunula is placed it is appropriate to have two statues representing adoring angels. In the middle of the Ostensorium here should be a receptacle of such a size that a large Host may be easily put into it; care must be taken that the Host does not touch the sides of this receptacle. On the front and back of this receptacle there should be a crystal, the one on the back opening like a door, when closed, the latter must fit tightly. The circumference of this receptacle must either be of gold or, if of other material, it should be gilt and so smooth and polished that any particle that may fall from the Host will be easily detected and removed. The lunula must be inserted and recovered without difficulty, hence the de need for keening it in an upright position should be construed with this end in view. The ostensorium need not necessarily be blessed, but it is better that it should be. The form "Benedictio tabernaculi" (Rit. Rom., tit. viii, xxiii) or the form "Benedictio ostensorii" (Rit. Rom., in Appendice) may be used. When carried to and from the altar it ought to be covered with a white veil.
The lunula (lunette) is made of the same material as the ostensorium. If it be made of any material other than gold, it must be gilded (Cong. Sac. Rit., 31 August, 1867). In form it may be either of two crescents or of two crystals encased in metal. If two crescents be used, the arrangement should be such that they can be separated and cleaned. Two stationary crescents, between which the Sacred Host is pressed, are, for obvious reasons, not serviceable. If two crystals are used it is necessary that they be so arranged that the Sacred Host does not in any way touch the glass (Cong. Sac. Rit., 14 January, 1898). The ostensorium, provided it contains the Blessed Sacrament, may be placed in the tabernacle, but then it should be covered with a white silk veil. (Recent authors say that since the ostensorium is intended merely ad monstrandam and not ad asservanduam SS. Eucharistiam it should not be placed in the tabernacle.) When the Blessed Sacrament is taken out of the ostensorium after Benediction it may or may not be removed from the lunula. If it is removed it should, before being placed in the tabernacle, be enclosed in a receptacle, called the repository (custodia, repositorium, capsula), which is made like the pyx, used in carrying Holy Communion to the sick, but larger, and may have a base with a very short stem. If the Blessed Sacrament be allowed to remain in the crescent-shaped lunula both It and the lunula may be placed in the same kind of receptacle, or in one specially made for this purpose, having a device at the bottom for keeping the Sacred Host in an upright position. The latter may have a base and short stem, and a door, which fits tightly, on the back part, through which the lunula is inserted. This receptacle is made throughout of silver or of other material, gilt on the inside, smooth and polished, and surmounted by a cross. Nocorporal is placed in it. If the lunula be made of two crystals, encased in metal, it may, when containing the Blessed Sacrament, be placed in the tabernacle without enclosing it in a custodia. If the host be placed before the Consecration in the lunula made of two crystals, the latter must be opened before the words of Consecration are pronounced. The lunula and custodia are blessed with the form "Benedictio Tabernaculi" (Rit. Rom., tit. viii, xxiii) by a bishop or by a priest having the faculty. They lose their blessing when they are regilt, or when they become unfit for the use for which they are intended. All the sacred vessels, when not actually containing the Blessed Sacrament, should be placed in an iron safe, or other secure place, in the sacristy, so as to be safeguarded against robbery or profanation of any kind. Each ought to be placed in its own case or covered with a separate veil, for protection against dust and dampness.[1]
[1] Written by A.J. Schulte. Transcribed by Michael C. Tinkler. The Catholic Encyclopedia, Volume I. Published 1907. New York: Robert Appleton Company. Nihil Obstat, March 1, 1907. Remy Lafort, S.T.D., Censor. Imprimatur. +John Cardinal Farley, Archbishop of New York