Some years ago I found an ordinary looking toggle switch at K-Mart, the kind you have in your home for lighting, but it had 3 positions: off, low and high. I found what it did was insert a diode in series for the low position. Clever.
Two items: 1) anyone seen these lately? 2) would the low setting somehow shorten the life of the bulb by only conducting half the time, thereby causing the filament to repeatedly cool and heat, stressing it thermally and/or physically? OR would it lengthen the life by only burning for half the time? Or would it not make any difference at all?
I'd build a test rig, but, given the variability of bulbs, it would probably take hundreds of trials to establish a pattern.