164 POLARIZATION OF LIGHT. 



air, are, for the ordinary ray, sin r- t K - 0'605, and' for 



I'DOo 



the extraordinary ray, sin r = TTOT = 0*674 ; and the cor- 



responding angles are 37 14' and 42 23'. Hence, if the 

 length of the compound prism be so adjusted that the ray 

 shall fall upon the bounding surface at an incidence interme- 

 diate to these two angles, the ordinary ray will be wholly 

 reflected, while the extraordinary ray will be transmitted. 

 It is obvious that the length of the rhomb which will give 

 this incidence is much shorter than in Nicol's prism. 



(178) M. Haidinger has observed a remarkable pheno- 

 menon of polarized light, by which it may be recognised by 

 the naked eye, and its plane of polarization ascertained. This 

 phenomenon consists in the appearance of two brushes, of a 

 pale orange-yellow colour, the axis of which coincides always 

 with the trace of the plane of polarization ; these are accom- 

 panied, on either side, by two patches of light, of a comple- 

 mentary or violet tint. In order to see them, the plane of 

 polarization of the light must be turned quickly from one 

 position to another, so as to shift the position of the brushes. 

 Thus they may be observed by looking for a few moments 

 at one of the images of a circular aperture, formed by a 

 rhomb of Iceland spar, and then at the other, and so alter- 

 nately. They gradually disappear when the eye continues 

 directed to them in the same position ; but they may be 

 made to reappear by shifting that position, or the plane of 

 polarization on which it depends. 



The most probable explanation of this phenomenon seems 

 to be that given by M. Jamin, in which it is ascribed to the 

 refracting coats of the eye. When polarized light falls upon 

 a pile of parallel plates, the proportion of the refracted to the 

 incident light varies with the plane of polarization, being a 



