Abstract: A thermally controlled microscope stage assembly for use in measuring transformation temperatures between -180.degree. and +600.degree.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.