116 Rev. H. Lloyd on the Phenomena of Light 



the cone just mentioned. The remote cone then disappeared, 

 and the circular arch diminished; and, as the obliquity of the 

 line to the axis was further increased, these two luminous por- 

 tions merged gradually into the two pencils into which a sin- 

 gle ray is divided in the other parts of the crystal. 



The same experiments were repeated by bringing the flame 

 of the lamp close to the first surface of the crystal. In this 

 case the lens was removed, and the incident cone of rays 

 formed by covering the surface of incidence with a thin me- 

 tallic plate perforated with a minute aperture. The results 

 were perfectly similar to those obtained in the former case. 



But to apply a yet more palpable test to this theory, I 

 substituted a narrow linear aperture for the point, in the plate 

 next the lamp ; and fixed it so that the plane passing through 

 the line on the first plate and the point in the second, should 

 be the plane of the optic axes. In this case, according to the 

 received theory, all the rays transmitted through the two aper- 

 tures 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 Hamilton's beautiful deduction from the same 

 theory, the ray proceeding in the direction of an optic axis 

 should be refracted in every plane passing through that line. 

 In accordance with this conclusion I found, on looking through 

 the second aperture, that the luminous line was undilated, 

 except in the direction of one of the optic axes ; and that in 

 the neighbourhood of this direction its boundaries ceased to 

 be rectilinear, and it swelled out into an oval curve. 



This experiment seems to go far in affording a general veri- 

 fication of the principle. I was anxious, however, to observe 

 the emergent cone more directly. After some trials I effected 

 this with the sun's light, and received the rays, emerging 

 from the aperture in the second plate, on a screen of rough- 

 ened glass. I was thus enabled to observe the phenomenon 

 at various distances, and with all the advantages of enlarge- 

 ment. The light was sufficiently bright, and the appearance 

 distinct, when the plane section of the cone of rays on the 

 screen was even two inches in diameter. 



Examining the emergent cone with a tourmaline plate, 1 

 was surprised to observe that one radius only of the section 

 of the cone vanished, in a given position of the axis of the 

 tourmaline ; and that the ray which disappeared ranged 

 through 360°, as the tourmaline plate was turned through 

 180°. From this it appeared that all the rays of the cone are 

 polarized in different planes. 



On examining this curious phenomenon more attentively, 



