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



axes, which are inclined to each other at an angle of 38°. 

 The middle line bisecting this angle is parallel to the axis of 

 the prism, and hence stands axial or equatorial, according as 

 the prism is suspended from its acute or its obtuse angle. 

 The position of the middle line is therefore a function of the 

 point of suspension, varying as it varies ; at one time support- 

 ing the law of Plucker, and at another time contradicting it. 

 Heavy spar is positive. 



Sulphate of Strontia [Ccelestine) , — This is also a positive 

 crystal, its form being precisely that of heavy spar ; the only 

 difference is, that, in the case before us, the optic axes inclose 

 an angle of 50° instead of 38°. The corroboration and con- 

 tradiction of heavy spar are exhibited here also. 



Sulphate of Zinc. — Suspend the crystalline prism from its 

 end, and mark the line which stands equatorial when the mag- 

 net is excited. A plate taken from the crystal parallel to this 

 line, and to the axis of the prism, on examination with polar- 

 ized light, will display the ring systems surrounding the ends 

 of the two optic axes. The middle line, therefore, which bi- 

 sects the acute angle inclosed by these, stands axial. It ought, 

 however, to stand equatorial, for the crystal is negative. 



Sulphate of Magnesia. — Suspending the crystalline prism 

 from its end, and following the method applied in the case of 

 sulphate of zinc, we discover the ring systems and the position 

 of the middle line. This line stands axial, but the crystal is 

 negative. 



Topaz. — This being one of the crystals pronounced by M. 

 Plucker as peculiarly suited to the illustration of his new law, 

 it is perhaps on that account deserving of more than ordinary 

 attention. In the letter to Mr. Faraday, before alluded to, 

 M. Plucker writes : — 



" The crystals most fitted to give the evidence of this law 

 are diopside (a positive crystal), cyanite, topaz (both negative), 

 and others crystallizing in a similar way. In these crystals 

 the line (A) bisecting the acute angles made by the two 

 optic axes, is neither perpendicular nor parallel to the axis 

 (B) of the prism. Such a prism, suspended horizontally, will 

 point neither axially nor equatoriaily, but will take always a 

 fixed intermediate direction. This direction will continually 

 change if the prism be turned round its own axis (B). It may 

 be proved by a simple geometrical construction, which shows 

 that during one revolution of the prism round its axis (B), 

 this axis, without passing out of two fixed limits C and D, 

 will go through all intermediate positions. The directions C 

 and D, where the crystal returns, make, either with the line 

 joining the two poles, or with the line perpendicular to it, on 



