and their relation to Crystalline Form and to the Optic Axes. 455 



systems, and commence with those which have as fundamental 

 form the rhombohedi-on or regular hexagonal prism, the first in 

 the series being tourmaline. The examples experimented wth 

 were magnetic ; the crystal is negative, and its optical axis was 

 repelled. Calcareous spar comes next ; and as the experiments 

 with this crystal bear upon the points of difference discussed by 

 M. Pliicker in his introduction, we give them entire. 



" Iceland spar exhibits the action of the magnetic axis in a veiy 

 decided manner. A fragment of any form, the dimensions of 

 which are not too great in comparison with the distance of the 

 pointed poles from each other, sets itself always so that its optic 

 axis falls in the equatoreal plane. A plate cut perpendicular to 

 the axis, the diameter of which is three or fom- times greater 

 than its thickness, when hung between the polar points so as to 

 be able barely to swing between them, sets itself like a diamag- 

 netic body ; by raising the plate a little, or by increasing the 

 distance which separates the poles, the plate turns as if its optic 

 axis were repelled. This result is always the same if the spar 

 be chemically pm-e ; with impure crystals other cii'cumstances 

 come into play. 



" Dm-ing the fonner experiments a crystal was found which was 

 magnetic, but whose axis, notwithstanding, was repelled in the 

 same manner as the transparent spar. And this fact was one of 

 those which led to the opinion, that the magnetic action which 

 caused the optic axis to recede was independent of the magnetism 

 or diamaguetism of the mass of the crystal. 



" Messrs. Tyndall and Knoblauch assert, on the contrary, that 

 the magnetic spar sets itself between the poles as if its optic axis 

 were attracted; and base upon this their fundamental notion, 

 that the deportment of the crystal between the poles when the 

 mass is magnetic is the reverse of that exhibited when the crystal 

 is diamagnetic. The same idea had been already expressed in 

 the mechanical theory of crystalline deportment mentioned in 

 the introduction. For the explanation of the matter, we obtained, 

 through the kindness of Dr. Krantz, crystals of the spar of 

 various forms and from different localities. 



" The very first ciystal which we examined was an opake homo- 

 geneous mass readily cleavable, and obtained from New York; 

 it was diamagnetic, and behaved itself correspondingly between 

 the poles. A little raised, however, it turned and set its axis in 

 the plane of the meridian. A plate cut perpendicular to the 

 axis corroborated this attraction of the axis. 



" The same deportment was exhibited by a semitransparent 

 specimen taken from a solid mass obtained at Prague. 



"One of the remaining crystals, a regular hexagonal prism from 

 Andreasbitfg, exhibited the deportment above alluded to aa being 



