J. V. HAHFOED — POLARISATION OF LIGHT. 155 



a crystal under a definite system, and it is not venturing far into 

 the domain of speculation to imagine that the law is that of attrac- 

 tion and repulsion ; or, in other words, polarity of some kind, as 

 exemplified in the ordinary magnetic property. Thus, if the archi- 

 tecture of crystals is assumed to be governed by a species of 

 polarity, certain phenomena of light which are intimately connected 

 with crystallography may not inappropriately borrow or adopt the 

 same term. 



Before leaving this part of the subject some further peculiar 

 properties of the doubly refi-acted beam must be noticed. 1st. 

 When the beam passes directly along the axis of the crystal, i.e., 

 the line connecting the obtuse angles of the rhomb, it is not 

 divided. This again seems to denote the symmetry of arrangement 

 and equal and uniform density in the molecular arrangement along 

 that line. 2nd. In every other direction (increasing to a maximum 

 at what may be termed the equator of the crystal) a separation 

 of the rays takes place. One of these, called the ordinary ray, 

 is refracted in accordance with the law established by Snell, 

 viz., that refraction takes place in the plane of incidence of the 

 light, and that the sine of the angle of refraction bears a constant 

 relation to the sine of the angle of incidence. A reference to Fig. 5 

 will make this matter more intelligible. The refraction of the 

 other I'ay, called the extraordinary ray, does not obey this law. It 

 is not refracted constantly in the plane of incidence, and its index 

 of refraction is not constant. The index for the ordinary ray is 

 1-654; that for the extraordinary ray is r483 to 1-654. The 

 refractive power of the crystal is therefore greatest in the direction 

 traversed by the ordinary ray, again proving that the molecular 

 arrangement is more dense in that direction than it is in the other. 

 The greater refraction of the ordinary ray is at once observed by 

 placing a crystal of the spar over a piece of paper on which a black 

 spot or other mark is made. One spot of the two, resulting from 

 double refraction, which seems to be raised to a higher level than 

 the other, is the ordinary ray. 



This difference between the refractive powers of the crystal in 

 relation to the ordinary and extraordinary rays is applied very 

 ingeniously in the constniction of the optical instrument so well 

 known as Mcol's prism. By reference to Fig. 6, it is seen that an 

 elongated prism of Iceland spar is cut diagonally in its length 

 through the two obtuse angles, and the two parts are then re-united 

 with Canada balsam. The refractive power of this stands between 

 those of the ordinary and the extraordinary ray. A ray of ordinary 

 light, therefore, impinging on the first surface of the prism is as 

 usual split into two in traversing the first portion of the crystal. 

 The extraordinary ray, passing from a medium of less density into 

 the Canada balsam, which exceeds it in refractive power, is bent in 

 the usual way inwards, then issues from the balsam into the spar, 

 and pursues its course throiigh the second half of the prism and 

 finally issues in the polarised state. The ordinaiy ray is altogether 

 reflected out of the prism, because it passes fi'om the greater re- 



