268 



PROGRESS IN MICROSCOPY 



The stage is heated by a resistor located at E and F. The temperature 

 is measured by a thermometer at //, its tip being very close to the 

 specimen. Figure 11.2 shows a hot stage (type Kofler of Reichert), 

 fitted to a microscope. The more widely used types of the stages can 

 be used up to 350° C. Observation is always carried out at a low 



Fig. ll.l. Cross-section through the Kofier W to 350" hot stage. 



magnification and, therefore, objectives with a long working distance 

 can be employed thus preventing the ill effects caused by the temperature 

 rise. The objectives generally employed have a x 3, x 5 magnification, 

 X 10 at most, associated with eyepieces x 10 or x 20. Therefore, 

 total magnification does not exceed x 200. Except, of course, during 

 observations, the objective is kept away as far as possible from heated 

 specimens in order to prevent any damage. When investigating sub- 

 limation, A is substituted for a cap covered with a glass plate which 

 serves as condenser for the substance to be sublimated. Special 

 devices allowing to study low pressure sublimation have been devised. 

 Cold stages whose minimal temperature is under (fQ are valuable 

 for measuring the melting point and studying the polymorphism of 

 organic compounds which are liquid at normal temperatures. 



