Purpose of Air Gap in Gas Lasers

[gildomar]

What's the purpose of having an air gap in certain gas lasers? Just to be clear, I'm talking about the air filled space between the lasing gas (with glass separating the two), and the partially silvered mirror at the end that let's the light out. Why isn't the partially silvered mirror just at the end of the gas tube?

 

[flatmaster]

The glass behind the mirrors needs to be machined and polished with great precision into a mirror. You wouldn't be able to do this at the bottom of a long, blown glass tube.