imperceptible
/ˌɪmpəˈseptəbl; ˌɪmpɚ`sɛptəbl/ adj
that cannot be noticed or felt because so small, slight or gradual （因细小、 轻微、 渐变）觉察不出的, 感觉不到的, 些微的
an imperceptible change in temperature 温度的难以察觉的变化
an almost imperceptible shift of opinion 几乎觉察不到的意见上的改变.