In a way that can be known or perceived; in a manner that is capable of being recognized.
From cognisable (able to be known) plus the adverbial suffix -ly. This follows standard English patterns for converting adjectives to adverbs.
This adverb is extremely rare in modern English—most writers would simply say 'noticeably' or 'perceptibly' instead, showing how some grammatically valid words fade from use!
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