REFLECTED-LIGHT MICROSCOPY 159 



anisotropic surface, in many substances, shows a double reflection 

 that varies as does the wave-length of the incident hght. This is bi- 

 reflective dispersion and characteristic colours are observed. To com- 

 plement the resulting data the analyser is crossed with the polarizer. 

 Rotating the sub-stage causes the anisotropic surface to exhibit 4 

 directions at right angles along which extinction still obtains. Intensity 

 is highest at 45° from such directions. 



Let us now set the sub-stage so that the "main" directions lie 45° 

 from the incident-vibration direction. Owing to two different reflect- 

 ances correlating the two main directions, the incident vibration 

 rotates about a small angle. Let OX and O 7 be the two main directions 

 of the anisotropic surface and OP the amplitude of the incident linear 

 vibration (Fig. 4.39). OP may then be decomposed in the two vibra- 

 tions OX and OY directed according to the two main directions. 

 After being reflected, the vibrations OX and OF do not have the 

 same amplitude since reflectances are always below unity. Owing 

 to the difference between the reflectances correlating OX and OY, 



^ r~T — " 



Fig. 4.39. After being reflected, the incident vibration OP becomes OP'. 



the latter become 0X[ and OY^ after being reflected by the object. 

 If no phase change occurs after the reflection, 0X[ and OY[ exhibit 

 the linear vibration OP' which has rotated about a small angle in 

 relation to OP. To every main direction corresponds not only a specific 

 reflectance but a specific phase change as well. As phase changes are 

 not the same for both main directions, the incident linear vibration 

 is converted after being reflected, in an elliptic vibration. Generally, 

 the ellipse is substantially flattened and its major axis differs but little 

 from the direction of the incident vibration. 



As the angle of rotation depends on the reflectance ratio of 

 the main directions, the bi-reflective dispersion generates rotations 

 which are not the same at all wave-lengths. Rotating the analyser 



