PURPOSE: To use part of a plane mirror as a magnifying glass like a concave mirror by attaching a heater at one corner of the back side of the plane mirror.

CONSTITUTION: Heater 2, with heat generating body 7 arranged in a concentric cicle from the center toward the outer circumference, is attached at a lower corner on the back side of a commercially available plane mirror. This kind of mirror 3 is ordinarily used as a smooth plane mirror but if heater 2 is switched on, only the back surface expands before the front side expands because of the difference in heat conductivity, thus making a concave mirror partially. Therefore, this part can be used as a magnifying glass.