If something can be seen with the naked eye, it can be seen without the help of an instrument.

This organism is too small to be seen with the naked eye. 这种生物体太小，用肉眼观察不到。

The police found traces of blood on his jacket that were invisible to the naked eye. 警方在他的夹克衫上发现了肉眼观察不到的血迹。