A thermally controlled microscope stage assembly for use in measuring transformation temperatures between -180° and +600°C. The stage assembly includes a thick-walled platen with a resistance heater in an annular cavity adjacent a specimen chamber and a cooling channel arranged along the periphery of the platen for, respectively, heating and cooling the specimen chamber. The platen has a lighting well disposed along the optical axis of the associated microscope. A condenser is built in the lighting bore and includes a pair of axially spaced plano- spherical fused quartz lenses both with their spherical surface facing the incident illuminating beam. The lens closer to the microscope has its plane surface, which defines its isophote plane, level with the upper surface of the platen. The specimen chamber is defined by the top surface of the platen, a thick-walled annular member seated on the platen and a thin transparent plate member. The temperature in the specimen chamber is measured by a platinum resistance probe which forms one arm of a linearizer bridge. A heating relay and a cooling relay are connected respectively in series with heating and cooling supplies and a thermostat control selectively controlling one of the relays in response to the sign of the voltage difference between the output voltage of the linearizer bridge and a set voltage determined by an adjustable voltage source corresponding to the set temperature.
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