The Collaborative International Dictionary of English v.0.48:

Magnifier \Mag"ni*fi`er\, n.
   One who, or that which, magnifies.
   [1913 Webster]

WordNet (r) 3.0 (2006):

magnifier
    n 1: a scientific instrument that magnifies an image

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "magnifier":
   achromatic lens, astigmatic lens, burning glass, camera,
   coated lens, concave lens, concavo-convex lens, condenser,
   convex lens, eyeglass, eyepiece, glass, hand lens, lens,
   magnifying glass, meniscus, object glass, objective,
   objective prism, ocular, prism, reader, reading glass,
   telephoto lens, toric lens, varifocal lens, zoom lens