unperceivable

/ʌnpərˈsivəbəl/
adjective
  1. Not able to be seen, heard, felt, or noticed; impossible to perceive.
    • The changes in the painting's color over time were so gradual they were almost unperceivable.
    • To the naked eye, the movement of the clock's hour hand is unperceivable.
    • The difference in temperature was unperceivable without a thermometer.
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