270 



PROGRESS IN MICROSCOPY 



heat is developed or heat absorption occurs at the weld (Peltier effect). 

 For instance, if heat is being developed, the heat will be absorbed 

 if the current direction is reversed. 



Fig. 11.3. O.P.L. (Optique Precision de Levallois) hot stage microscope designed for 



investigating opaque objects. 



3. CALIBRATING THE HOT STAGE 



The hot stage is heated by a resistor. The current is rheostat- 

 adjusted. Calibration consists in determining, for every rheostat ad- 

 justment, the temperature variation versus time. 



The procedure is the following: the hot stage (with its ancillaries 

 and a specimen) is set on the microscope, the thermometer being fully 

 inserted into its recess. The rheostat-adjusted potential difference is 

 shown by a voltmeter connected across the hot stage terminals. Then, 

 the rheostat slide is so adjusted that, initially, the potential difference 

 is low, e.g. 30 volts. The temperature is recorded every minute until 

 the temperature rise drops to less than TC per minute. This shows, 

 therefore, in relation to the potential difference considered, tempera- 

 tures for which the rise is 1°, 2°, 3°, ... per minute. The foregoing 

 measurements are resumed for several potential differences, and curves, 



