Mr. Luoyp on Conical Refraction. 147 
cluded that these planes touched the wave-surface only in the two points just men- 
tioned ; and, consequently, that a single ray, incident upon a biaxal crystal in such a 
manner that one of the refracted rays should coincide with an optic axis, would be 
divided into two, determined by the points of contact. ‘This result, if verified by 
experience, would place a remarkable distinction between the phenomena of uniaxal 
and biaxal crystals ; but though the case was examined by M. Bior, no corresponding 
appearances were observed. 
Professor Hamiiron has shown that the four planes of which we have spoken touch 
the wave-surface, not in two points only, but in an infinite number of points, consti- 
tuting each a small circle of contact, whose plane is parallel to one of the two circular 
sections of the surface of elasticity ; and that, consequently, a single ray of common 
light, incident externally in the above-mentioned direction, should be divided within 
the crystal into an infinite number of refracted rays, constituting a conical surface. 
Here, then, are two singular and unexpected consequences of the undulatory theory, 
not only unsupported by any facts hitherto observed, but even opposed to all the 
analogies derived from experience. If confirmed by experiment, they would furnish 
new and almost convincing proofs of the truth of that theory ; and if disproved, on the 
other hand, it is evident that the theory must be abandoned or modified. 
Being naturally anxious to submit the undulatory hypothesis to this delicate test, and 
to establish or disprove these new results of theory, Professor HamiLron requested me 
to institute a series of experiments with that view. I accordingly applied myself to this 
interesting research with all the attention which the subject so well merited, and have 
fortunately succeeded in verifying both cases of conical refraction. The substance | 
employed in these experiments was arragonite, which is well known to be a biaxal 
crystal, whose axes are inclined at an angle of nearly 20°. I selected it partly on 
account of the magnitude of its biaxal energy, and partly also because the optical 
elements of this mineral have been determined, apparently with great care, by Professor 
Rupsere ; and therefore the results of theory could be applied to it at once without 
further examination. ‘The specimen I used was one of considerable size dnd purity, 
procured for me by Mr. Dotonp, and cut with its parallel faces perpendicular to the 
line bisecting the optic axes. 
The first-mentioned species of conical refraction, it has been observed, takes place 
mm air, when a ray of common light is transmitted within the crystal in the direction 
of the line joining two opposite cusps of the wave. If we suppose such a ray to pass 
in both directions out of the crystal, it is evident that it must emerge similarly at both 
surfaces ; consequently, the rays which are transmitted along this line within the 
crystal, and form a diverging cone at emergence at the second surface, must be inci- 
dent in a converging cone at the first. Having therefore nearly ascertained the required 
direction by means of the system of rings in polarized light, I placed a lens of short 
