Which preposition to use with ignotum

per Occurrences 3%

It is evidently meant as a sportive sally of ridicule on Johnson, whose style is thus imitated, without being grossly overcharged: 'It is easy to foresee, that the idle and illiterate will complain that I have increased their labours by endeavouring to diminish them; and that I have explained what is more easy by what is more difficult ignotum per ignotius.

Which preposition to use with  ignotum