It's invisible to the naked eye.

因为它是肉眼不可见的。

His brush stroke so subtle as to be invisible to the naked eye.

他的笔触非常微妙，肉眼看不见。

Using a telescope, Galileo discovered stars that were invisible to the naked eye.

【invisible】伽利略用望远镜发现了肉眼看不到的恒星。

Through his telescope he could see millions of stars that were invisible to the naked eye.

【naked，nakedness，nakedly】通过望远镜他可以看见数以百万计肉眼看不见的星星。

So how could I prove the existence of something that was invisible to the naked eye?

那么如何证明肉眼看不见的东西的存在呢？

When you handed me back the card, what you didn't see was I looked at it so fast it was invisible to the naked eye.

当你把名片还给我的时候，你没有看到的是我看它的速度快到肉眼看不见。

In fact that was one of Galileo's first and most important discoveries: stars that were invisible to the naked eye were easily seen through his telescope, even though it only had a lens a few centimeters across.

事实上，这是伽利略最早也是最重要的发现之一：肉眼看不见的星星通过他的望远镜就很容易被看到，尽管它只有一个几厘米宽的透镜。