REFLECTED-LIGHT MICROSCOPY 



161 



cemented elements A and B, is set in the illuminator. The light origi- 

 nated by the vertical illuminator is reflected by the ad face which acts 

 as a mirror. The elements A and B are cemented with a substance 

 whose index is such that only the ordinary ray is reflected by the 

 surface ab. The extraordinary ray is transmitted and absorbed by the 

 darkened face be. After being reflected on the object /*, the ordinary 

 ray is again fully reflected by ab and, if the object is not anisotropic, 

 no light reaches the eyepiece Oo. The device works as would two 

 perfectly-crossed polarizers. If the object is anisotropic, the same 

 effects as described previously occur. To obtain the equivalent of an 

 analyser rotation, a quarter-wave plate is inserted between the ob- 

 jective and the Forster prism. Owing to the double passage, the 

 quarter-wave plate brings about the same effects as a half-wave plate 

 and the incident vibration is rotated without being altered. 



6. SCRUTINIZING OF HOT-METAL SURFACES 



Scrutinizing structure changes taking place at high temperatures, 

 such as re-crystallization, are often desirable. Usually, to prevent 

 oxidation, the sample is put in vacuo or in an actionless atmosphere. 

 The chamber comprises a fused quatz optical peep-hole through which 

 the sample is observed. To protect the microscope optical system, 



\Q/o. 



Fig. 4.41. Dyson long working distance attaciiment. 



the chamber walls are cooled. The objective's working distance must 

 be such as to allow focusing within the chamber. Owing to their 

 long working distance, mirror-objectives are particularly suitable for 

 this work (cf. Chapter I, § 3). A spherical-surface objective, of nu- 

 merical aperture us'mu = 65, provides a working distance extending 



