REFLECTED-LIGHT MICROSCOPY 



131 



Stray rays are originated from at least 2 successive glass reflections 

 which weaken them markedly. In metallographic microscopes, stray 

 rays are originated from a single reflection and, hence, more intense. 



Fig. 4.4. Stray light reflected by the lenses of the objective. 



The device, shown in Fig. 4.2, does away with a substantial part of 

 the stray hght, which is intercepted by the mirror M. However, its 

 drawbacks were evinced elsewhere. 



The greater part of the stray light is virtually originated from the 

 reflection in the central part of the objective's lenses. Leitz observed 

 this and developed a device reducing stray hght materially. When 

 using this device, illumination is originated solely from the objective's 

 periphery. This annular illumination increases the microscope's sensi- 

 tivity in observation of micro-relief. Another device, perfected by 

 Zeiss, makes use of polarized light. The polarizer £Pi and the analyser fp2 

 are placed ahead of G and of the eyepiece, respectively. The polarizer 

 5Pi is so arranged that the vibration it transmits is either at right angles 

 to or in the incidence plane SIJ related to G. Under such conditions, 

 the reflection on G does not alter anything and the vibration remains 

 straight after impinging on G. If the analysers fP., and iPi are crossed, 

 the light reflected by the lenses of the objective Ox is fully occluded. 

 Therefore, there is no stray light any longer. But the light, reflected 

 by the object P, is occluded, too, and must re-appeai. In the method 

 propounded by Zeiss (Fig. 4.5), the quarter-wave plate Q is placed 

 against the objective and, being gone through twice, becomes half-wave. 

 The vibration, reflected by P, rotates 90° and may then pass through iPo- 

 Nomarski suggested using, instead of a half-wave plate, a parallel- 

 to-the-axis quartz plate, whose thickness is adequate to show a chan- 



