350 Mr. Potter on Conical Refraction. 



conical refraction. In this instance, however, the law was 

 anticipated from theory by Professor Hamilton." 



With the exception of the last sentence but one, this de- 

 scription gives a correct and clear account of the appearances, 

 but under a condition which Professor Lloyd has omitted to 

 notice. The luminous ring is seen, and seen perfectly only, 

 when the lens is so placed in its distance from the crystal, 

 that what he calls the two rays, are, in fact, the two virtual 

 images of the luminous point on the first surface. The posi- 

 tion of these virtual images within the crystal is found by the 

 formulae of geometrical optics, their distance from the second 

 surface, when the incidence is nearly perpendicular, being 

 , thickness of the plate 



equal to r- — r- — ~^~j ""• 



^ retractive index 



When the eye-lens has any other distance than that which 

 gives the distinct images of the aperture, we find the appear- 

 ances changed, and that the bright ring is not the section of 

 a cylinder of equal diameter at all distances from the crystal, as 

 Professor Lloyd supposes he has made out, by means of sun- 

 light. That he should have overlooked phaenomena so inter- 

 esting, can be accounted for only by his devotion to his theory, 

 which has thus robbed him of a fine discovery. 



Whether we draw the ei/e-lens away from the crystal, or push 

 it nearer, wejind similar appearances, and we see that the ring 

 was formed by the intersection of two cones {or more correctly 

 of two series of cones), the one converging so as to have the 

 bright spot, we formerly found on the second surface, for its 

 vertex, and the other diverging, and having the ring, formerly 

 found on the second surface, for its base. I say more correctly 

 two series of cones, for we formerly found that the bright spot 

 only gradually became fainter. The same appearances are 

 seen in pushing the lens nearer the crystal, for gradually one 

 portion of the light of the ring increases in diameter, whilst 

 the other diminishes, and a bright centre is formed, which 

 comes to its smallest magnitude, and then diverges again into 

 a solid cone of rays, having a bright centre which gradually 

 fades away. The phaenomena show that the cones, the one 

 virtual as being beyond the second surface of the crystal, 

 measured from the eye, and the other real as being formed 

 in the emergent light, are in reality a series of cones having 

 their vertices on opposite sides of, and at different distances 

 from, the ring. When the rays of either series of cones are 

 collected into their smallest space forming the bright spot, this 

 is evidently only an approximate effect ; and when the inci- 

 dent pencil is very small, this spot is seen evidently outside 

 the crystal, rather than on the second surface for one series. 



