Morel d'Arleux's explanation goes
roughly as follows:
Système de Lebrun
sur la Physionomie
(1 : 3)
Human heads seen in front and
profile views, drafted for the demonstration of Le Brun's system
Figure 1A (centre) The horizontal line runs
through each corner of the eye, forming an angle of which the
opening gives the measure of the genius of men, ... those of whom
nature has tempered the passions by a soft moderation in according
them the continual use of reason and the habitual exercise of
all amiable virtues.
Figure 1B (right) The raised inner corners
... designate beings gifted with genius, moved by noble and generous
passions, and destined by their fine deeds to deserve immortality.
Figure 1C (left) Individuals subject
to shameful, detestable, or atrocious passions are recognizable
by the lowering of the inner corner.