How do you call the instrument...?

How do you call the instrument which lets submarines look on the water surface?

A periscope, right?


Than how do you call the instrument that doctors use to listen to your heartbeat?

That's right, a stethoscope.

And what is the name of the instrument astronomers use to watch the stars?

Oh yeah, a telescope.

Now what do we call the instrument with which we can see through walls?

[Hover for answer](/s "a window")