Nor did they lay small stress upon certain accessaries of their art, such as the thrones, which they composed in the richest and most ostentatious manner; and the landscapes, which they drew with an astonishing degree of truth from nature, besides the architecture frequently constructed in the forms of porticos or tribunes. It may sometimes be observed, also, that adapting themselves to the workmanship and to the design of the altar, they feigned a continuation of it within the painting, so that by the resemblance of colour and of taste, the eye is deceived, the illusion produced rendering it doubtful where the exterior ornament[27] terminates, and where the picture begins. We ought not, therefore, easily to give credit to certain writers who have undervalued the merits of such masters, pronouncing their labours mechanical, as those of mere practical artificers, inasmuch as Serlio is known to have supplied several of them with architectural designs.[28] We ought rather to subscribe to the opinion of Daniel Barbaro, whose extensive learning did not prevent him, in his work entitled _Pratica di Prospettiva_, from expressing his admiration of them, even from the commencement, as follows: "In this art they left many fine remnants of excellent works, in which we behold not only landscapes, mountains, woods, and edifices, all admirably designed; but even the human form, and other animals, with lines drawn to the eye, as if to a centre placed in the most exact perspective. But in what manner, and by what rules they proceeded, no author of whom I am aware, has left any account to instruct us."