490 HISTORY OF OPTICS. 



as to form a conical pencil. For the direction of 

 the refracted ray is determined by a plane which 

 touches the wave surface, the rule being that the 

 ray must pass from the center of the surface to the 

 point of contact; and though in general this contact 

 gives a single point only, it so happens, from the 

 peculiar inflected form of the wave surface, which 

 has what is called a cusp, that in one particular 

 position, the plane can touch the surface in an 

 entire circle. Thus the general rule which assigns 

 the path of the refracted ray, would, in this case, 

 guide it from the center of the surface to every 

 point in the circumference of the circle, and thus 

 make it a cone. This very curious and unexpected 

 result, which Professor Hamilton thus obtained from 

 the theory, his friend Professor Lloyd verified as an 

 experimental fact. We may notice also, that Pro- 

 fessor Lloyd found the light of the conical pencil to 

 be polarized according to a law of an unusual kind ; 

 but one which was easily seen to be in complete 

 accordance with the theory. 



8. Fringes of Shadows. The phenomena of the 

 fringes of shadows of small holes and groups of 

 holes, which had been the subject of experiment by 

 Fraunhofer, were at a later period carefully ob- 

 served in a vast variety of cases by M. Schwerd of 

 Spires, and published in a separate work 17 , Beu- 



17 Die Beugungs-ersc&einnngen, atis dem Fundamental-gesctz 

 der Undulations Theorie analytisch enttvickelt und in Bildern 

 dargestellt, von F. M. Schwerd. Mannheim, 1835. 



