12 



CONICAL REFRACTION. 



It is easy to render an account of these various appearances. 

 When the aperture mn (fig. 3) is at all considerable, the rays cm, 

 en, proceeding to its circumference from a point on the first surface, 

 will be sensibly inclined to the cusp-ray, which we shall suppose to 

 be'the line co, connecting the point on the first surface with the 



Fig. 3. 



centre of the aperture. Consequently the interior refracted rays, 

 mq, nr, as well as the exterior, mp, ns, will be inclined outwards ; 

 and it is obvious that there will be a central bright space, limited 

 by the lines mq, nr, each point of which will be illuminated by one 

 interior and one exterior ray. The light in this space, therefore, will 

 have double the intensity of that of the surrounding space ; and 

 as the rays which combine to form it are polarized in planes at 



right-angles to'one another, the resulting light will be unpolarized. 

 When the aperture is diminished, the inclination of the rays mq, 

 nr, to one another is lessened, until finally they are reduced to 

 parallelism, and the central bright space contracts to a point. 

 This is represented in (fig. 4). When the aperture is still further 



