On Permanent Magnetism. 309 



optic axes of the mica, but reversed in character according as 

 the x of the selenite crosses or is parallel to the mica z. This 

 is the simplest analysis I can give, and it follows on conside- 

 ration from Norremberg's data. Here are two such prepara- 

 tions, in each of which four selenites are alternated between 

 five \\ micas. In the first the modified lemniscates are wider 

 apart than in the mica, but the resulting fringes originate 

 approximately in the mica axes. In the second the selenites 

 are at right angles with their former position, and the fringes 

 still centre in the axes, but the curves are reversed ; and the 

 resulting "palm-tree" pattern is perhaps one of the most 

 beautiful, both in colour and figure, which it is possible to 

 behold. 



The few other preparations here are built up of either four 

 or six ternary elements constructed on Norremberg's system, 

 each consisting of two parallel micas, with a selenite between 

 them either crossed or parallel. In the first you readily 

 recognize the " palm-tree " character of the last figure, 

 " crossed." It is needless to describe the others ; for here, 

 too, variety is boundless ; but I purposely reserve for the last, 

 two combinations composed of exactly the same arrangements 

 of both mica and selenite, and all the micas the same (^X) 

 thickness, but the selenite films in one slightly thicker than in 

 the other. The difference in effect is purposely not so great 

 as to prevent your recognizing the same general figure in 

 both, but is still conspicuous and interesting. Let me, in 

 conclusion, hope that the beauty of these preparations con- 

 structed after Norremberg's method, and the facility with 

 which they can be prepared, may make them better known. 



[At the conclusion of the paper Mr. Wright described and 

 exhibited an adaptation to the microscope by Messrs. Swift 

 and Son, by the aid of which all the preparations and crystals 

 requiring highly convergent light could be shown on the stage 

 of any microscope constructed with a draw-tube.] 



XLY. On Permanent Magnetism. 

 By K. H. M. Bosanquet. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN a former paper I have proposed to follow out Faraday's 

 analogy between magnetism and electricity; and the fol- 

 lowing is a summary of the resulting point of view. 



Magnetic induction, or the number of lines of force through 

 unit area, is the quotient of magnetomotive force by resistance, 

 in magnetic circuits. 



