imperceptibly

The light faded imperceptibly as the sun began to set.

(Chinese) 随着太阳开始落山，光线悄无声息地（极细微地）暗了下来。

The robot's movements have improved imperceptibly over hundreds of trials.

(Chinese) 经过数百次试验，机器人的动作有了细微到难以察觉的改进。