[Milsurplus] Mystery Tube Update

[David Stinson]

The prism has no electrical connection, so I theorize
that it's there as an optical block to allow one to 
see the ionized gas without the glare of the filament.  
But unless the device was intended to be viewed 
at an angle, the filament is not placed for optimum
optical shielding.  And it would seem a simple disc
would work- why a prism?  
At first, I thought it might be a "flicker tube," which
is an early electro-mechanical way to have a 
"special effect" for a sign (my years in Las Vegas
got me thinking of it), but that doesn't address the
odd choice of a prism.  Mystery indeed.