different between imperceptibly vs instantaneous

English

imperceptible +? -ly

imperceptibly (comparative more imperceptibly, superlative most imperceptibly)

Not noticeably; too small to be detected; too little to be perceived.
No one noticed that he had moved imperceptibly backwards.

From New Latin [Term?].

instantaneous (not comparable)

Occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time. [from 17th c.]
Synonyms: immediate, instant; see also Thesaurus:instantaneous

Webster, Noah (1828) , “instantaneous”, in An American Dictionary of the English Language
instantaneous in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.
“instantaneous” in Dictionary.com Unabridged, Dictionary.com, LLC, 1995–present.
"instantaneous" in Cambridge Advanced Learner's Dictionary (Cambridge University Press, 2007)
"instantaneous" in Compact Oxford English Dictionary, (Oxford University Press, 2007)
Oxford English Dictionary, second edition (1989)

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