There is no threshold for the incandescent lamp, after all its an analog
device and light output is varied from zero to 100% continuous with respect
to varying the voltage. You don't see it because you haven't enough current
to heat the filament up - see if you could see it in the dark. I'm sure if
you have a 6V car or motorcycle battery instead of the C cells, the 12v lamp
would light up.

It will light but just barely. Remove one cell from a two cell
flashlight and you'll see.