1540s, "one who or that which enlarges," agent noun from magnify. Specifically as "a magnifying lens" by 1660s.
Entries linking to magnifier
magnify v.
late 14c., magnifien, "to speak or act for the glory or honor (of someone or something)," from Old French magnefiier "glorify, magnify," from Latin magnificare "esteem greatly, extol, make much of," from magnificus "great, elevated, noble," literally "doing great deeds," from magnus "great" (from PIE root *meg- "great") + combining form of facere "to make, to do" (from PIE root *dhe- "to set, put").
Meaning "increase the apparent size of by use of a telescope or microscope" is from 1660s, said to be a development peculiar to English. Related: Magnified; magnifying. Magnifying glass is by 1650s.