Source : Webster's Revised Unabridged Dictionary (1913)

Observable \Ob*serv"a*ble\, a. [L. observabilis: cf.F.
   observable.]
   Worthy or capable of being observed; discernible; noticeable;
   remarkable. --Sir. T. Browne.

         The difference is sufficiently observable. --Southey.
   -- {Ob*serv"a*ble*ness}, n. -- {Ob*serv"a*bly}, adv.

Source : WordNet®

observably
     adv : in an imperceptible manner or to an imperceptible degree;
           "the power of the Secretary of State in London
           increased gradually but imperceptibly" [syn: {imperceptibly},
            {unnoticeably}] [ant: {perceptibly}]