POLARIZATION OF LIGHT. 165 



minimum when that plane coincides with the plane of inci- 

 dence, and a maximum when it is perpendicular to it. These 

 variations are nothing at a perpendicular incidence : they are 

 greatest when the angle of incidence is equal to the angle 

 of polarization. Accordingly, when the polarized light is 

 incident obliquely on the plates, the refracted light should 

 exhibit two dark brushes, enlarging from the centre to the 

 circumference, in the plane of polarization ; and two bright 

 brushes in the perpendicular plane. 



In the preceding explanation, the incident light is sup- 

 posed to be homogeneous. When white light is used, the inten- 

 sities of its several components, in the refracted pencil, will 

 vary with the refractive indices, and consequently the brushes 

 will be coloured. M. Jamin has shown that the effect of a 

 single refracting surface will be to produce two yellow brushes, 

 whose axis is in the plane of polarization. 



