(a.) Not to be perceived by the senses; not discernible\; not easily apprehended.

atomic behind the curtain behind the scenes camouflaged concealed corpuscular disguised embryonic ephemeral evanescent faint fugitive germinal granular hidden ill-defined impalpable imponderable inappreciable inaudible inconsiderable inconspicuous indiscernible indistinct indistinguishable infinitesimal insensible insignificant intangible invisible latent microcosmic microscopic minuscule minute molecular momentary obscure out of sight secret sightless slight subatomic submerged subtle tenuous thin tiny trivial ultramicroscopic unapparent unappreciable unbeheld unbeholdable unclear undiscernible undistinguishable unnoticeable unnoticed unobservable unobserved unperceivable unperceived unrealized unseeable unseen unviewed unwitnessed vague viewless