

430 Miscellaneous Intelligence. 



There was no " central point" of emanation, though a majority perhaps, 

 of the whole, appeared to originate in the directions of M Cassiopceia" 



and M Ursa Minor." 



None were seen of startling brilliancy, though many were exceed- 

 ingly beautiful. We should have continued our observations until day- 

 dawn, had not the light of the moon interfered. On the night of the 

 11th also, the meteors were more than usually abundant, but by no 

 means so numerous as on the evening preceding. 



(2.) Mineral Point, Wise. — A notice in the Tribune, signed " Sper- 

 ry," and dated Mineral Point, August 9, 1849, states that seventy-eight 

 shooting stars were seen there in "over an hour," probably before 

 midnight, seventy-three of which appeared to start from a point near 

 the Swan, a little S.E. of the zenith, and passed off in a southwesterly 

 direction. The number of observers is not mentioned. 



V. Miscellaneous Intelligence. 



1. On the Magnetic Relations of the Positive and Negative Optic 

 Axes of Crystals ; by Professor Plucker of Bonn, in a letter to Dr. 

 Faraday, (Phil. Mag., [3], xxxiv, 450.) — Allow me, sir, to communi- 

 cate to you several new facts which, 1 hope, will spread some light 

 over the action of the magnet upon the optic and magnecrystallic axes. 

 I. The first and general law I deduced from my last experiments 

 is the following: — u There will be either repulsion or attraction of 

 the optic axes by the poles of a magnet, according to the crystalline 

 structure of the crystal. If the crystal is a negative one, there will be 

 repulsion; if it is a positive one, there will be attraction." 



The crystals most fitted to give the evidence of this law are diopside 

 (a positive crystal), cyanite, topaz (both negative), and other ones, 

 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 crystal, suspended 

 horizontally like a prism of tourmaline, staurotide, or "red ferridcyanid 

 of potassium," in my former experiments, will point neither axially 

 nor equatorially, 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 both sides of these lines, nngles equal to the 

 angle included by A and B; the first being the case if the crystal is a posi- 

 tive one, the last if a negatire one. Thence it follows, that if the crys- 

 tal by any kind of horizontal suspension should point to the poles of a 

 magnet, it is a positive one ; if it should point equatorially, it is a nega- 

 tive one. This last reasoning conducted me at first to the law men- 

 tioned above. 



The magnecrystallic axis, I think is optically speaking the line bisect- 

 ing the (acute) angles made by the two optic axes; or in the case of 

 one single axis, this axis itself. The crystals of bismuth and arsenic 



i 



