authentic (especially as having given rise to an Italian proverb), and
it has also great point and value. I translate Vasari's words
literally.

"This work (his paintings in the Campo Santo of Pisa) acquired for
him, both in the city and externally, so much fame, that the Pope,
Benedict IX., sent a certain one of his courtiers into Tuscany, to see
what sort of a man Giotto was, and what was the quality of his works,
he (the pope) intending to have some paintings executed in St.
Peter's; which courtier, coming to see Giotto, and hearing that there
were other masters in Florence who excelled in painting and in mosaic,
spoke, in Siena, to many masters; then, having received drawings from
them, he came to Florence; and having gone one morning into Giotto's
shop as he was at work, explained the pope's mind to him, and in what
way he wished to avail himself of his powers, and finally requested
from him a little piece of drawing to send to his Holiness. Giotto,
who was most courteous, took a leaf (of vellum?), and upon this, with
a brush dipped in red, fixing his arm to his side, to make it as the
limb of a pair of compasses, and turning his hand, made a circle so
perfect in measure and outline, that it was a wonder to see: which
having done, he said to the courtier, with a smile, 'There is the
drawing.' He, thinking himself mocked, said, 'Shall I have no other
drawing than this?' 'This is enough, and too much,' answered Giotto;
'send it with the others: you will see if it will be understood.' The
ambassador, seeing that he could not get any thing else, took his
leave with small satisfaction, doubting whether he had not been made a
jest of. However, when he sent to the pope the other drawings, and the
names of those who had made them, he sent also that of Giotto,
relating the way in which he had held himself in drawing his circle,
without moving his arm, and without compasses. Whence the pope, and