118 Rev. H. Lloyd ow M* Phenomena of Light 



-anBii_9i3w fes ~ /UaufJ 



and R,= 13° 54' 49". Now the difference of these angles, or 



3° 0' 38", may be called the angle of the cone; and half the sum, 

 or 15° 25' 8'', is the mean angle of emergence. The angle 

 15° 58', found above, differs from this by 33' only ; but the ob- 

 served angle of the cone is about double of that given by theory. 



I also measured the angle of the cone by receiving it on a 

 screen of roughened glass at different distances, and tracing 

 the outline of the section on the screen : the diameter of this 

 section and the distance being then measured, the angle was 

 determined. Three measurements taken in this manner gave 

 for the magnitude of that angle respectively 6° 24', 5° 56' 9 

 6° 22', the mean of which, 6° 14>', agrees very nearly with that 

 determined by the former method. 



Conceiving that the difference between experiment and 

 theory arose chiefly from the rays which were inclined to the 

 optic axis all round at small angles, and which were transmit- 

 ted at the second surface in consequence of the sensible mag- 

 nitude of the aperture, I determined to try the effects of 

 apertures of various forms and dimensions. 



When the aperture was at all considerable, two concentric 

 circles were seen to surround the optic axis, the interior circle 

 having about double the brightness of the annulus which sur- 

 rounded it. The light of the interior circle was unpolarized, 

 while that of the surrounding annulus was polarized accord- 

 ing to the law already mentioned. When the aperture was 

 diminished, the inner circle contracted in diameter, the breadth 

 of the outer annulus remaining nearly the same; until the 

 former was finally reduced to a point in the centre of the ex- 

 terior circle. When the aperture was still further diminished, 

 a dark space sprung up in the centre, which enlarged as the 

 aperture decreased ; until finally, with a very minute aperture, 

 I succeeded in rendering this space about f of the whole, or 

 of reducing the breadth of the luminous annulus to about £ of 

 its exterior diameter. 



With this diminished aperture I examined the appearance 

 produced by a line of light on the first surface parallel to the 

 plane of the optic axes. The swelling curves, which it has 

 been already remarked, surrounded the optic axis in this case, 

 were reduced to a breadth corresponding to that of the an- 

 nulus in the former experiment, and were separated by a 

 considerable dark interval. When the plane passing through 

 the two apertures deviated a little from the plane of the optic 

 axes, the phenomena underwent many beautiful changes, the 

 curves assuming in all cases the form of the conchoid, whose 

 pole was the projection of the optic axis, and asymptote the 

 line on the first surface. 



