Might be a thermistor, a special resistor that is designed to be a high resistance when cold, and when you turn the light switch on, it gets hot and the resistance drops to a low value. This reduces the turn on inrush current on the lightbulb, reducing the stress on the filament, and after the thermistor warms up, the bulb is running at around the 10% reduction that your eyes won't notice.
Half wave diodes will make the lightbulb much dimmer and the light a lot browner.
Thermistors are commonly used in switching power supplies in computers, used to reduce inrush current upon power up.
