chapters belonging to this Section bear the title "Prospettiva"
_(see Nos._ 869 _and_ 870_), this being the term universally applied
at the time to Optics as well as Perspective (see Vol. I, p._ 10,
_note to No._ 13, _l._ 10_)_.
_At the beginning of the XVIth century the Ptolemaic theory of the
universe was still universally accepted as the true one, and
Leonardo conceives of the earth as fixed, with the moon and sun
revolving round it, as they are represented in the diagram to No._