Are bulbs brighter in a series or parallel curcuit?

Batteries in series makes the voltage additive. If the bulb is only rated at a specific voltage and you double the voltage the bulb will glow brighter but its life span will be shortened. Batteries in parallel will keep the voltage at the same level as a single battery but the endurance drain of the batteries will be doubled. Example, if a battery is drained of power, with a bulb being left on continuously, in one hour then two batteries in parallel would allow the bulb to glow for two hours before the batteries were drained of power.