The science of Astronomy was so closely joined in those times with the art of Astrology, that it is difficult to draw a clear distinction between them as they were then understood. The one was but the practical application of the other, and in common use their names were often confused and used interchangeably. We are not surprised then to find the title of Imperial Astrologer given to Michael Scot in the colophon to his _Astronomia_; he was sure to be employed in this way, and the fact will help us to determine with probability what was the _first_ book he wrote for the Emperor, that to which the _Liber Particularis_ was a sequel. For there is actually extant under Scot’s name an astrological treatise bearing the significant name of the _Liber Introductorius_.[53] This title agrees exceedingly well with the position we are now inclined to give it, and an examination of the preface confirms our conjecture in a high degree. It commences thus: ‘Here beginneth the preface of the _Liber Introductorius_ which was put forth by Michael Scot, Astrologer to the ever August Frederick, Emperor of the Romans, at whose desire he composed it concerning astrology,[54] in a simple style[55] for the sake of young scholars and those of weaker capacity, and this in the days of our Lord Pope Innocent IV.’[56] One cannot help noticing the close correspondence between this and the colophon of the _Astronomia_. The two treatises were the complement each of the other. They must have been composed about the same time, and were doubtless meant to serve as text-books to guide the studies of Frederick’s youth. That this royal pupil should have been led through astrology to the higher and more enduring wonders of astronomy need cause no surprise, for such a course was quite in accordance with the intellectual habits of the age. It may be doubted indeed whether the men of those times would have shown such perseverance in the observations and discoveries proper to a pure science of the heavens, had it not been for the practicable and profitable interest which its application in astrology furnished. Astronomy, such as it then was, formed the last and highest study in the Quadrivium.[57] It was here that Scot had carried off honours at Paris, and now in his _Liber Introductorius_ and _Astronomia_, we see him imparting the ripe fruits of that diligence to his royal charge, whose education, so far as regarded formal study, was thereby brought to a close.