162 POLARIZATION OF LIGHT. 



emergent light is never perfect, and that its intensity is much 

 weakened by absorption both the rays being absorbed in 

 their passage through the crystal, although with unequal 

 energies. 



The polarization produced by double refraction is the 

 most complete of any ; while the intensity of the polarized 

 pencils is greater than in any other case, being very nearly 

 one-half of the intensity of the original light. The intensity 

 of the light reflected from a plate of glass, at the polarizing 

 angle, is not more than the one-twelfth part of that of the 

 incident light. 



(176) From the foregoing it would appear that the most 

 effective mode of producing a polarized ray of strong inten- 

 sity is, to transmit a ray of common light through a rhomb 

 of Iceland spar, and to shut off one of the two refracted 

 pencils by means of an opaque plate on' the second surface of 

 the rhomb. Here however there is a difficulty. The two 

 rays will overlap, unless the aperture by which the light is 

 admitted to the first surface of the rhomb is very small, or 

 the thickness of the rhomb is very considerable. The former 

 condition reduces the quantity of the light ; the latter is 

 difficult of attainment, on account of the rarity of large 

 specimens of the crystal of sufficient purity. 



These difficulties may be evaded, by receiving the light 

 upon a lens of short focus, placed immediately behind the 

 rhomb. The rays which have passed through the lens and 

 the rhomb will converge to two foci, one of which may be 

 covered by a diaphragm, and the rays diverging from the 

 other suffered to proceed alone. These rays may be reduced 

 to parallelism, when required, by receiving them upon a 

 second lens, placed at its own focal distance from the point 

 of divergence. The aperture of this lens will determine the 

 breadth of the beam. It will be convenient to employ a 



