444 WRIGHT— POLARIZED LIGHT IN THE 



such methods are ascertained. It is shown in the general case that 

 vertically incident, plane-polarized light waves become on reflection 

 elliptically polarized (as a result of a difference in phase between 

 the components parallel and normal to the plane of incidence) ; that 

 the amplitude of the component of the light vector in the plane of 

 incidence is different from that normal to the plane of incidence. 



In special cases where crystallographic symmetry relations pre- 

 scribe the positions of the principal axes of refringence and of 

 absorption, as in isotropic, uniaxial, and the principal planes of ortho- 

 rhombic crystals, the above relations are simplified to the extent that 

 the refracted waves are plane-polarized and not elliptically polarized 

 as in the general case ; as a result, normally incident, plane-polarized 

 light waves whose vibration directions are either parallel or normal 

 to the plane of symmetry are reflected as plane-polarized waves ; but 

 the intensity of the two reflected waves is different because of the 

 difference in the refractive and absorption indices parallel and 

 normal to the plane of symmetry. Therefore normally incident, 

 non-polarized light contains after reflection a certain amount of 

 plane-polarized light and this amount increases with the strength of 

 the birefringence and of the biabsorption in the crystal plate. 



The presence of plane-polarized light in essentially non-polarized 

 light can be detected by'any one of the well-known physical methods, 

 such as are commonly used in determinations of sky polarization. 

 Of these methods Koenigsberger adopted the Savart plate with 

 rotating glass compensator ; a second method is suggested above 

 which employs either a single calcite cleavage plate with proper ap- 

 erture (after the manner of the Haidinger lens or the Pickering 

 photometer) or a small portable Koenig-Martens photometer. This 

 method is superior to the first method in two respects : it is simpler 

 in adjustment and in manipulation; it is based on a photometrically 

 better principle, namely contrast of two or a series of illuminated 

 fields, the first at minimum intensity, the adjacent field at maximum 

 intensity with a sharp line of demarcation which disappears when 

 the intensity of illumination in the adjacent fields is the same. The 

 attainable accuracy depends on the least perceptible difference in in- 

 tensity of illumination which the eye can detect; this is about 1.5 to 

 2 per cent, under ordinary conditions of illumination. An opaque 



