of Crystalline Bodies between the "poles of a Magnet. 179 



We do not see any possibility of referring this election of a 

 particular diagonal to the influence exerted by the optical axis, 

 at least in the case of beryl. 



To ascertain the exact nature of this influence, we had re- 

 course to discs, cut so that the optical axis of the crystal lay 

 as a diameter in the plane of each. In this way the influence 

 of mere form was totally annulled, and the pure action of the 

 optical axis, if such existed, might be observed. 



Our first discs, five in number, were taken from a semi- 

 transparent crystal of Iceland spar, and lay at various angles 

 to the sides thereof. In ail these cases the law of Pliicker 

 was strictly verified, the optical axis being always repelled. 



Four discs and one square were next taken from two trans- 

 parent crystals of the spar, and suspended successively between 

 the poles. We were by no means prepared for the reply 

 given to these experiments ; in each of the five cases the optical 

 axis set itself distinctly axial. 



The balance sheet of our inquiries up to the present time is 

 this : out of eleven crystals of Iceland spar examined as above, 

 five have obeyed the law of Plucker, while six have contra- 

 dicted that law. 



In determining whether the optical axis will be repelled or 

 not, it is not necessary to cut the crystals in the manner de- 

 scribed. A thin rhomb cloven from the crystal and ground 

 into the shape of a disc, will decide the question. If it belong 

 to the class whose optical axis is repelled, the line bisecting 

 the acute angles of the rhomb will set itself axial ; if to the 

 other class, the same line will set itself equatorial. 



Discs thus prepared form undoubtedly the purest means of 

 investigating this question. The rhomb itself, however, with- 

 out being ground into a disc, affords us sufficient intelligence 

 as to the class of the crystal to which it belongs. Is its optical 

 axis repelled, then the long diagonal of the rhomb will incline 

 to the axial position ; is the optical axis attracted, the long 

 diagonal will stand nearly equatorial. 



The same adherence of the diagonal to the axial or equa- 

 torial position continues after a thin bar containing the dia- 

 gonal has been severed from the rhomb. For example, a 

 bar containing the short diagonal of that class whose optical 

 axis is attracted, will stand nearly axial. This fact is perhaps 

 worthy of notice : if Iceland spar be diamagnetic, as Prof. 

 Plucker asserts, and if the optical axis be repelled, what is it 

 that overcomes the united action of both in the case of this 

 diagonal ? The projection of the optical axis lies in the same 

 direction as the bar ; both therefore work together, and both 

 strive, in virtue of the two properties mentioned, to attain the 



N2 



