[Footnote 87: Cf. Villani's Statistics of Florence, and Corio's of Milan.] Since this was the nature of academical instruction in the humanities before the age of printing, it followed that the professor had a direct interest in frequently shifting his scene of operations. More than a certain number of such books as I have just attempted to describe could not be carried in his head. After he had dictated his work on the 'Georgics' at Florence, he was naturally anxious to move to Milan and to do the same. A new audience gave new value to his lectures, and another edition, as it were, of his book was put in circulation. In the correspondence which passed between professors and the rectors of the high schools previously to an engagement, we sometimes find that the former undertake to explain particular authors during their proposed residence. On these authors they had no doubt bestowed the best years of their lives, making them the vehicle for all the miscellaneous learning they possessed, and grounding their fame upon the beauty, clearness, and copiousness of their exposition.[88] [Footnote 88: For humorous but vivid pictures of a professor's lecture-room, see the macaronic poems of Odassi and Fossa quoted by me in vol. v. of this work.] Having described the conditions under which professorial teaching was conducted in the fifteenth century, it is now of some importance to form a notion of the state of the book market and the diffusion of MSS. before the invention of printing. Difficult as it is to speak with accuracy on these topics some facts must be collected, seeing that the high price and comparative rarity of books contributed in a very important degree to determine the character of the instruction provided by the humanists.