Why are narrow discharge tubes preferred for viewing spectra?

[NitroNinja]

This is a part of a homework question, and a concept that is expected to be understood. "Why do you think discharge tubes have a narrow waist?".

I was thinking about attempting the question like this:
Discharge tubes are mainly used to view spectra. And hence a thin tube, will allow a thin emission of light. But then, i couldn't think of an explanation.
I'd appreciate help!

 

[Simon Bridge]

You use a slit to get a thin line for spectra. This works for any shape "tube".
It's used on stars for instance.

You'll notice also that commercial florescent lighting usually comes in long thin tubes... sometimes bent into spirals for domestic light fittings. All they have to do is let you see stuff.

So what you need is some advantage to having the gas confined to narrow containers.