6 CONICAL REFRACTION. 



which presented itself when this disposition was complete was in 

 the highest degree curious. There appeared at first a luminous 

 circle, with a small dark space in the centre ; and in this dark 

 central space were two bright points, separated by a narrow and 

 well-defined dark line. These appearances are represented in 

 (figs, a and b). When the aperture in the plate was slightly 



Fig. a. Fig. 



shifted, the phenomena rapidly changed, assuming in succession 

 the forms represented in (figs, c, d, e). In the first stage of its 

 change, the central dark space became greatly enlarged, and a 

 double sector appeared in the centre. The circle was reduced to 

 about a quadrant, and was separated by a dark interval from the 

 sector just mentioned. This is represented in (fig. c.) The 

 remote sector then disappeared, and the circular arch diminished, 



Fig. c. Fig. d. Fig. e. 



as in (fig. d) ; and as the inclination of the internal ray to the 

 cusp-ray was further increased, these two luminous portions 

 merged gradually into the two pencils, into which a single ray is 

 divided in the other parts of the crystal. This change is repre- 

 sented in (fig. e). 



Similar observations were made without the lens, by bringing 

 the flame of the lamp near the first surface of the crystal, and 

 forming the converging cone by covering that surface also with a 

 thin metallic plate, perforated with a minute aperture. In this 

 case the line connecting the two minute apertures was adjusted 

 as before, and the phenomena were the same as in the former 



