It's stupid question time again!
In some glass tubes (vacuum tubes) I noticed that the glass in some parts of the tube was covered with some silvery material. And there always was a metal ring under this area. At first I thought it was shielding or something, but then I saw it in a special incandescent tube, whose only purpose was to emit light, so I assumed, that that would not need shielding.
Why is the extra layer on the glass there? Do I assume correctly, that it was done using that ring (probadly had some chemical substance in it)?
An example: At the very top of this http://upload.wikimedia.org/wiki... tube is an example of what I mean.