(_a_ _b_) is equal to (_c_ _d_).

[Footnote: See Pl. VII, No. 3. Reference may also be made here to
two pen and ink drawings of heads in profile with figured
measurements, of which there is no description in the MS. These are
given on Pl. XVII, No. 2.--A head, to the left, with part of the
torso [W. P. 5a], No. 1 on the same plate is from MS. A 2b and in
the original occurs on a page with wholly irrelevant text on matters
of natural history. M. RAVAISSON in his edition of the Paris MS. A
has reproduced this head and discussed it fully [note on page 12];
he has however somewhat altered the original measurements. The
complicated calculations which M. RAVAISSON has given appear to me
in no way justified. The sketch, as we see it, can hardly have been
intended for any thing more than an experimental attempt to
ascertain relative proportions. We do not find that Leonardo made
use of circular lines in any other study of the proportions of the
human head. At the same time we see that the proportions of this
sketch are not in accordance with the rules which he usually
observed (see for instance No. 310).]

The head _a_ _f_ 1/6 larger than _n_ _f_.