Some believe that the image perceived in this way takes form on the surface of the mirror, but they are mistaken. The diviner looks at this surface fixedly until it disappears, and a curtain, like a mist, is interposed between him and the mirror. Upon this curtain are designed the forms he wishes to see, and this permits him to give indications, either affirmative or negative, concerning the matter on which he is questioned. He then describes his perceptions as he has received them. The diviners, while in this state, do not see what is really to be seen (in the mirror); it is another kind of perception, which is born in them and which is realized not by sight but by the soul. As to the character and quality of the crystal to be used, Abbot Tritheim, the master of the famous Cornelius Agrippa, says:[276] Procure of a lapidary a good, clear, pellucid crystal of the bigness of a small orange,—_i.e._, about one inch and a half in diameter; let it be globular, or round each way alike; then you have got this crystal fair and clear, without any clouds or specks. Get a small plate of pure gold to encompass the crystal round one-half; let this be fitted on an ivory or ebony pedestal. Let there be engraved a circle round the crystal; afterwards the name: Tetragrammaton. On the other side of the plate let there be engraved, Michael, Gabriel, Uriel, Raphael, which are the four principal angels ruling over the Sun, Moon, Venus, and Mercury. The four letters constituting the Tetragrammaton are the Hebrew characters _yôdh_, _hê_, _wâw_ and _hê_, יהוה. As this divine name was regarded in later Judaism as too sacred to be pronounced, the word lord, _adonai_, was substituted for it in the reading of the Scriptures. For this reason, when the vowel signs were added to the text to indicate the traditional pronunciation, the consonants Yhwh were provided with the vowels of _adonai_ and the name was therefore read Jehovah by Christian scholars. The Persian poet Jâmi writes thus of a magic mirror in the poem “Salamân and Absal”:[277] Then from his secret Art the Sage Vizyr A Magic Mirror made; a Mirror like The bosom of All-wise Intelligence, Reflecting in its mystic compass all Within the sev’nfold volume of the World Involved; and looking in that Mirror’s face The Shah beheld the face of his Desire. Roger Bacon (1214-1292) was probably the most gifted man of the thirteenth century, and his writings testify to an extraordinarily clear perception of the essential principles of scientific research. However, his true greatness was not generally appreciated in his own age, and popular fancy wove about his name a fabric of legend in which he appeared as an arch-necromancer and magician. The curious old work entitled “The Famous Historie of Fryar Bacon” gives a number of the strange recitals which became current in England in regard to Bacon’s wonderful powers.