422 



WRIGHT— POLARIZED LIGHT IN THE 



ference limen) for different intensities and different wave-lengths 

 shows that for an intermediate intensity of illumination differences 

 of intensity 1.6 to 2.0 per cent, can just be detected; for low (below 

 50 m.c.) and high (above 100,000 m.c.) intensities, the least per- 

 ceptible difference in intensity is greater. Under ordinary condi- 

 tions of illumination we may assume that 2 per cent, difference in 

 illumination intensity is about the limit perceptible to the eye. This 

 means that in order to be recognized by the eye there must be a dif- 

 ference of about 2 per cent, in intensity of light reflected by the two 

 components. To find the birefringence (Wa — ^1) required to pro- 

 duce this least perceptible difference in intensities of reflected light, 

 namely, 2 per cent., we may without sensible error use only the first 

 two terms of equation (31) and transform it to read 



712 



fix = 



0.02 (wi + i)(w2 — i) 



(35) 



from which the necessary birefringence (n^ — nj for the different 

 refractive indices n^ can be computed. These are listed in Table 5. 



TABLE 5. 



In this table is given the least degree of birefringence of a crystal plate 

 which on the reflection of vertically incident light produces a detectable differ- 

 ence in intensity of the two waves polarized at planes normal one to the other. 



This table shows that for very low-refracting, non-absorbing sub- 

 stances only a relatively slight increase in refractive index (bire- 

 fringence weak) is necessary to produce the required perceptible 

 difference in intensity ; for minerals of medium refringence and bi- 

 refringence, such as quartz, the difference limen increases, but the 

 birefringence required is still medium to weak ; for highly refracting 

 media the required difference in refractive indices increases rapidly. 

 The essential feature to note, however, is that minerals of medium 



