Now there is one more fact which must be explained in connection with crystallomancy. It has always been thought that the "subject"--that is, the boy or girl who looks into the stone, crystal, or ink-drop--must be absolutely innocent. The "subject" must be virtuous. In the Catholic Middle Ages the same idea took form especially in relation to the chastity of the "subject." Chastity was, in those centuries, considered a magical virtue. A maiden, it was thought, could play with lions or tigers, and not be hurt by them. A maiden--and the word was then used for both sexes, as it is sometimes used by Tennyson in his Idylls--could see ghosts or spirits, and could be made use of for purposes of crystallomancy even by a very wicked person. But should the subject have been secretly guilty of any fault, then the power to see would be impaired. The tragedy of Rossetti's poem turns upon this fact. In the poem a precious stone, of the description called beryl, is the instrument of divination. This beryl is round, like a terrestrial globe, and is supposed to be of the shape of the world. It is half transparent, but there are cloudings inside of it. Hidden among these cloudings are a number of evil spirits, who were enclosed in the jewel by magic. These spirits make the future appear visible to any virtuous person who looks into the stone; but they have power to deceive and to injure any one coming to consult them who is not perfectly chaste. The stone came from the East, and it was obtained only at the sacrifice of the soul of the person who obtained it. Having been brought to England, it became the property of a knightly family. This family consists only of a widow and her daughter Rose Mary. The daughter is in a state of great anxiety. She was to be married to a certain knight, who has not kept his affectionate promises. The daughter and the mother both fear that the knight may have been killed by some of his enemies. So they resolve to consult the beryl-stone. The mother does not know that her daughter has been too intimate with the absent knight. Believing that Rose Mary is all purity, the mother makes her the subject of an experiment in crystallomancy; and she looks into the beryl.