Both Uses

incredulous
in
Frankenstein - 1831 version
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"You may easily believe," said he, "how great was the difficulty to persuade my father that all necessary knowledge was not comprised in the noble art of book-keeping; and, indeed, I believe I left him incredulous to the last, for his constant answer to my unwearied entreaties was the same as that of the Dutch schoolmaster in The Vicar of Wakefield: 'I have ten thousand florins a year without Greek, I eat heartily without Greek.'†
p. 61.3

incredulous = unbelieving; or having difficulty accepting something so unexpected
The magistrate appeared at first perfectly incredulous, but as I continued he became more attentive and interested;
p. 203.1 *

incredulous = not believing

Definitions:

(1) (incredulous)
unbelieving; or having difficulty accepting something so unexpected
(2) (meaning too rare to warrant focus)