CONICAL REFRACTION. 15 



considerable dark interval, and the luminous line was prolonged 

 into this dark space, terminating abruptly near its centre. This 



Fig. i. 



appearance is represented in (fig. k). When the plate next the 

 eye was slightly shifted, so that the plane passing through the two 



Fig. *. 



apertures no longer coincided accurately with the plane of the optic 

 axes, the curves rapidly changed, preserving however, in all cases, 



Fig. I. 



the form of the conchoid, whose pole was the projection of the axis 

 of the emergent cone, and asymptote the line on the first surface 



Fig. m. 



ifigs. /, m). It is easy to show that these results are in accordance 

 with theory. 



