8 Messrs. J. Tyndall and H. Knoblauch on the 



Saxony. Aberdeen topaz we have not examined, but it also 

 is classed by Brewster among positive crystals. The obli- 

 quity of the middle line of topaz does not exist in the speci- 

 mens which have come under our notice ; it is exactly per- 

 pendicular to the principal cleavage, and consequently exactly 

 parallel to the axis of the prism. This agrees with the results 

 of Brewster, who found the optical axes to be " equally in- 

 clined to the plane of cleavage*." 



In experimenting with weak diamagnetic crystals, the greater 

 the number of examples the better; as, if local impurity be 

 present, it is thus more liable to detection. Our results with 

 heavy spar have been confirmed by ten different crystals ; with 

 ccelestine,by five; and with topaz, as has been stated, by seven. 

 The suspending fibre, in these and similar instances, was a 

 foot long and ~j of an inch thick, or about one-eighth of the 

 diameter of a human hair. 



Sugar. — It is well known that this crystal forms a prism 

 with six sides, two of which are generally very prominent, the 

 principal cleavage being parallel to these two, and to the 

 wedge-like edge which runs along the end of the prism. The 

 plane of the optical axes is perpendicular to the axis of the 

 prism, and their ends may be found by cutting out a plate 

 parallel to that axis, and inclined to the principal cleavage at 

 an angle of about 20°. Such a plate exhibits both ring 

 systems symmetrically, while a plate parallel to the principal 

 cleavage exhibits one system only. Suspended between the 

 excited poles, with the axis of the prism horizontal, and the 

 principal cleavage vertical, the plane of the optical axis stands 

 axial; according to the law of M. Pliicker, it ought to stand 

 equatorial, for the crystal is negative. 



Rock-crystal (Quartz). — This crystal has undergone more 

 than one examination by M. Pliicker, its deportment being, 

 "contrary to all expectation," very weak — a result, it may be 

 remarked, difficult of explanation on the hypothesis of an 

 " optic axis force." M. Pliicker's first experiments with this 

 crystal were apparently made with great exactitude, the cry- 

 stal being reduced to a spherical shape, and the influence of 

 mere form thus annulled. These experiments proved the 

 optical axis to be repelled. Later researches, however, in- 

 duced this philosopher to alter his opinion, and accordingly, 

 in his last memoirf, we find quartz ranked with those crystals 

 whose optical axes are attracted, with the remark "weak" 

 added parenthetically. We have not been able to obtain this 



* Lardner's Enc3 ; clopaedia, Optics, p. 204. 

 f Poggenclorff's Jnnalen, vol. lxxviii. p. 428. 



