The Collaborative International Dictionary of English v.0.48:

Microscopic \Mi`cro*scop"ic\, Microscopical \Mi`cro*scop"ic*al\,
   a. [Cf. F. microscopique.]
   [1913 Webster]
   1. Of or pertaining to the microscope or to microscopy; made
      with a microscope; as, microscopic observation.
      [1913 Webster]

   2. Able to see extremely minute objects.
      [1913 Webster]

            Why has not man a microscopic eye?    --Pope.
      [1913 Webster]

   3. Very small. Specifically, visible only by the aid of a
      microscope; as, a microscopic insect; also used
      figuratively; as, a microscopic advantage.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

microscopic
    adj 1: of or relating to or used in microscopy; "microscopic
           analysis"; "microscopical examination" [syn:
           microscopic, microscopical]
    2: visible under a microscope; using a microscope [syn:
       microscopic, microscopical]
    3: extremely precise with great attention to details; "examined
       it with microscopic care"
    4: so small as to be invisible without a microscope;
       "differences were microscopic" [syn: microscopic,
       microscopical]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "microscopic":
   3-D, atomic, close, constant, corpuscular, delicate, direct,
   embryonic, evanescent, even, exact, express, faithful, fine,
   germinal, granular, impalpable, imperceptible, imponderable,
   inappreciable, indiscernible, inerrable, inerrant, infallible,
   infinitesimal, intangible, invisible, mathematical, microcosmic,
   micrometrically precise, molecular, nice, pinpoint, precise,
   refined, religious, religiously exact, rigid, rigorous, scientific,
   scientifically exact, severe, square, stereoscopic, strict,
   subatomic, subtle, telescopic, tenuous, thin, three-dimensional,
   ultramicroscopic, undeviating, unerring, unseeable