Batteries in a series will provide a higher voltage to the bulb and it burns brighter. In parallel, the bulb receives a lower voltage and the batteries last longer.
Work Step by Step
Assume that each battery alone is a 1.5-V battery. With the batteries in series, 3 volts is provided to the lamp. It burns brighter than when the batteries are in parallel. With the batteries in parallel, the lamp is provided with 1.5 volts. It consumes only one-fourth the electrical power (because the current is halved, and power scales as the square of the current) of the other configuration, and is not as bright. However, because each battery supplies only half of this reduced current, the batteries last longer than when they are in the series configuration.