Great to see that it still works! What is the causing the blue glow towards the base of the lamp outside of the sodium discharge tube?

very awesome lamp! its interesting how its a vacuum insulated sodium lamp with a removable vacuum dewar jacket still, like sort of, inverted cross between an SO/H lamp and an SOI/H lamp, the Blue glow to me looks like the inner tube that the arc tube sits in has slightly had its vacuum go soft, similar to the sort of blue glow you get in a thermionic valve thats gone bad

Certainly looks like it's leaked a bit or some argon has diffused out the arc tube?

I guess the external light is the glow of argon. But I don't know the reason.