Both Uses

perceptible
in
Frankenstein - 1831 version
(Edited)

My father observed with pain the alteration perceptible in my disposition and habits and endeavoured by arguments deduced from the feelings of his serene conscience and guiltless life to inspire me with fortitude and awaken in me the courage to dispel the dark cloud which brooded over me.
p. 93.8

perceptible = noticeable
I indeed perceptibly gained on it, and when, after nearly two days' journey, I beheld my enemy at no more than a mile distant, my heart bounded within me.
p. 211.4 *

perceptibly = noticeably

Definitions:

(1) (perceptible)
capable of being noticed -- typically because it is different enough or large enough
(2) (meaning too rare to warrant focus)