You can put 60, 100 even 150 watt equivalent Compact Fluorescent bulbs in to replace a 40 watt incandescent bulb without overheating the fixture.

40 watt CFs use .075 amps or 9 watts

60 watters use .12 amps or 14watts

75 watters use .16 amps or 19 watts

100 watters use .19 amps or 22 watts

150 watters use .35 amps or 42 watts