154 Mr. Luioyp on Conical Refraction. 
1. Distance of screen from the aperture on the second surface of the crystal = 19.3 
half inches. Mean diameter of section of exterior cone = 1.27. Mean diameter of 
interior = 0.55. Corrected angle of cone thence computed = 2° 44. 
2, Distance of screen = 11.9. Mean diameter of section of exterior cone = 0.93. 
Mean diameter of interior = 0.41. Computed angle of cone = 3° 14’. 
The mean of these two measurements is 2° 59’. 
Inasmuch as the cusp-ray, within the crystal, corresponds to a cone of rays without, 
it is evident that there must be a converging cone incident on the first surface, equal 
to that which diverges from the second. With a view to determine its magnitude, J 
placed a kind of rough micrometer, consisting of two moveable metallic plates, imme- 
diately before the lens; and closed the plates until, on looking through the aperture 
on the second surface, I could see them touching the circumference of the annular 
section. The diameters of the interior and exterior circumferences of this section, 
at the distance of the lens, being thus ascertained, and the focal length of the lens 
measured, the corrected angle of the cone was found. ‘The mean of three measure- 
ments taken in this manner gave for this angle 3° 47’. But the methods by which 
this last result was obtained, do not seem susceptible of much accuracy. 
Before I conclude this part of the subject, I may observe that an interesting yaria- 
tion in the phenomena is obtained by substituting a narrow linear aperture for the 
small circular one, in the plate which covers the first surface of the crystal—that 
surface being close to the lamp. ‘The linear aperture is to be so fixed, that the plane 
passing through it and the aperture in the plate next the eye, shall be the plane of 
the optic axes. In this case, according to the received theory, all the rays trans- 
mitted through the two apertures should be refracted doubly in the plane of the optic 
axes, so that no part of the line should appear enlarged in breadth on looking through 
the second aperture ; whereas, according to Professor Hamitroyn’s beautiful conclu- 
sion from the same theory, the cusp-ray should be refracted in every possible azimuth. 
I found accordingly that the luminous line was undilated, except in the direction cor- 
responding to that of the cusp-ray ; and that in the neighbourhood of this direction 
its boundaries were no longer rectilinear, but swelled out in the form of an oval 
curve (fig. 7.) 
When a very minute aperture was used on the surface next the eye, in this experi- 
ment, the phenomenon was rendered much more remarkable. The swelling curves 
in this case were separated by a considerable dark interval, and the luminous line was 
prolonged into this dark space, terminating abruptly near its centre. ‘This appear- 
ance is represented in (fig. k.) When the plate next the eye was slightly shifted, so 
that the plane passing through the two apertures no longer coincided accurately with 
the plane of the optic axes, the curves rapidly changed, preserving, however, in all 
; 
