Dr. Wallis of Oxford, on one occasion, at night, in bed, proposed to himself a number of fifty-three places, and found its square root to twenty-seven places, and, without writing down numbers at all, dictated the result from memory twenty days afterward. It was not unusual with him to perform arithmetical operations in the dark, as the extraction of roots, _e. g._, _to_ forty _decimal_ places. The distinguished Euler, blind from early life, had always in his memory a table of the first six powers of all numbers, from one to one hundred. On one occasion two of his pupils, calculating a converging series, on reaching the seventeenth term, found their results differing by one unit at the fiftieth figure, and in order to decide which was correct, Euler went over the whole in his head, and his decision was found afterward to be correct. Pascal forgot nothing of what he had read, or heard, or seen. Menage, at seventy-seven, commemorates, in Latin verses, the favor of the gods, in restoring to him, after partial eclipse, the full powers of memory which had adorned his earlier life. The instances now given are mentioned by Mr. Stewart but perhaps the most remarkable instance of great memory in modern times, is the case of the celebrated _Magliabechi_, librarian of the Duke of Tuscany. He would inform any one who consulted him, not only who had directly treated of any particular subject, but who had indirectly touched upon it in treating of other subjects, to the number of perhaps one hundred different authors, giving the name of the author, the name of the book, the words, often the page, where they were to be found, and with the greatest exactness. To test his memory, a gentleman of Florence lent him at one time a manuscript he had prepared for the press, and, some time afterward, went to him with a sorrowful face, and pretended to have lost his manuscript by accident. The poor author seemed inconsolable, and begged Magliabechi to recollect what he could, and write it down. He assured the unfortunate man that he would, and setting about it, wrote out the entire manuscript without missing a word. He had a _local_ memory also, knew where every book stood. One day the Grand Duke sent for him to inquire if he could procure a book which was very scarce. "No, sir," answered Magliabechi; "it is impossible: there is but _one_ in the world; that is in the Grand Seignior's library at Constantinople, and is the _seventh book, on the seventh shelf, on the right hand as you go in_." VI. EFFECTS OF DISEASE ON THE MEMORY.