imperceptible
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.
What's the basis of your belief that the product is safe?
你凭什么相信产品是安全的?
However, he had at last been allowed to send a fax in which he informed the editor that the he had been arrested while counting the 1,084 steps leading to the fifteen-foot wall which surrounded the president's palace.