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):

microscopical
    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: so small as to be invisible without a microscope;
       "differences were microscopic" [syn: microscopic,
       microscopical]