1) Barely able to be perceived 2) Imperceptible 3) Not discernible 1) Imperceptible 2) Indistinguishable 3) Unperceivable 
• (a.) Not to be discerned; imperceptible; not discoverable or visible.
In`dis·cern'i·ble adjective [ Prefix in- not + discernible : confer French indiscernable .] Not to be discerned; imperceptible; not discoverable or visible. « Secret and indiscernible ways.» Jer. Taylor. -- In`dis*cern'i*ble*ness , 
[adj] - barely able to be perceived 2. [adj] - difficult or impossible to perceive or discern
insensible adjective barely able to be perceived; `the transition was almost indiscernible`; `an almost insensible change`
not discernible; that cannot be seen or perceived clearly; imperceptible.