 What we have here is just a glass tube with electrode on each end. Gases are evacuated out through this little tip of glass, and hydrogen is put back in, and then the glass is melted. So all we have in this glass tube is just hydrogen gas. Here we have a high voltage source. So we can get about 5,000 volts of electricity. Potential difference between these two electrodes. And when we do that, we excite that hydrogen electron. It moves to an outer energy level. And then since we're on 60 cycles per second, it's going to fall back and as it does, it emits energy. So we want to measure that. We measure that by using this piece of machinery. We have a slit on this end. You're going to look through here, and over on this side, you're going to see a virtual image with a little meter at the bottom so that you can measure. It looks like a little ruler. So what we're going to do is we're going to set this up so that the light from the hydrogen will pass through here. You'll see it here, and then you'll see the image reflected over here.