beam of electrons
Let's say it cooler: they were particle accelerators you pointed at your face.
Also, massive glass vacuum bottles that could implode if mishandled.
And with massive capacitors that could electrocute you if mishandled.
And heavy enough to seriously crush your foot (and possibly implode and electrocute you) if mishandled.
And the static electricity felt really neat on the back of your hand if you put it close to the screen.
They were so fucking cool, and we took them for granted, and now they're gone, and life is significantly less cool without them.