160 



PROGRESS IN MICROSCOPY 



(the polarizer and the sub-stage remaining stationary) will evince 

 seriatim a series of striking colours. 



In the foregoing observations, the reflectance of the main di- 

 rections, the colours, the eUipticity of the reflected vibration, the 

 angle of rotation and so forth, have provided valuable data pertaining 

 to pohshed metal surfaces. As already mentioned, the additional 

 equipment required is simple: merely setting a polarizer in the vertical 

 illuminator ahead of the 45°-slanted semi-reflective mirror and an 

 analyser between the latter and the eye is all that is required. To avoid 

 the birefringent superimposition effects, the objective's lenses are to 

 be tension-free. The 45°-slanted mirror has its lower surface (on 

 the objective side) coated with a semi-reflective layer so as to in- 

 crease the reflectance. Conversely, the upper surface (eyepiece side) 

 is processed to lower the reflectance thus preventing formation of 



TSP»- 



0. 



\Q/ 



Fig. 4.40. Foster device (Bausch and Lomb). 



a double image. Owing to the reflection of the 45'-slanted mirror, 

 when using two crossed polarizers, extinction is not uniform within 

 the whole field. In the Cooke, Troughton and Simms "Metalore" 

 microscope, the 45°-slanled mirror is in parallel light and extinction 

 is uniform within the whole field. The neat device, designed by Foster 

 (Bausch and Lomb) serves as 45°-slanted plate and polarizing device 

 at the same time (Fig. 4.40). A spar prism, consisting of the two 



