Both Uses of

illustrate

in

Frankenstein - 1831 version

As the circumstances of his marriage illustrate his character, I cannot refrain from relating them.
p. 33.4 *

illustrate = help make clear

…analysing all the minutiae of causation, as exemplified in the change from life to death, and death to life, until from the midst of this darkness a sudden light broke in upon me—a light so brilliant and wondrous, yet so simple, that while I became dizzy with the immensity of the prospect which it illustrated, I was surprised that among so many men of genius who had directed their inquiries towards the same science, that I alone should be reserved to discover so astonishing a secret.
p. 53.3

illustrated = helped make clear

Definitions:

(1) (illustrate as in: as illustrated by this example)
to help make clear -- typically by example
(2) (illustrate as in: an illustrated children's book)
to draw pictures (or provide photographs) to accompany a book or other writing
(3) (meaning too rare to warrant focus)