(a.) Imperceptible; not discoverable or visible\.

alike atomic behind the curtain behind the scenes camouflaged concealed corpuscular disguised embryonic evanescent germinal granular hidden impalpable imperceptible imponderable inappreciable indistinct indistinctive indistinguishable infinitesimal insensible intangible interchangeable invisible latent microcosmic microscopic molecular out of sight secret sightless standard stereotyped subatomic submerged tenuous thin ultramicroscopic unapparent unappreciable unbeheld unbeholdable undifferentiated undiscernible undiscriminated undistinguishable undistinguished uniform unnoticed unobservable unobserved unperceivable unperceived unrealized unseeable unseen unviewed unwitnessed viewless without distinction