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



themselves with the subject. But, if we except the illustrious 

 discoverer himself, there is no investigator in this branch of 

 science whose labours have been, to all appearance, so richly 

 rewarded as those of Professor Plikker of Bonn. 



In 1847 M. Pliicker had a magnet constructed of the same 

 size and power as that described by Mr. Faraday*, his object 

 being to investigate the influence of the fibrous constitution 

 of plants upon their magnetic deportment. While conducting 

 these experiments, he was induced to try whether crystalline 

 structure exercised an influence. " The first experiment," 

 says M. Pliicker, "gave an immediate and decided reply." 



Following up his investigations with crystals, he was led to 

 the affirmation of the following two laws: — 



" When any crystal whatever with one optical axis is brought 

 between the poles of a magnet '; the axis is repelled by each of the 

 poles; and if the crystal possess two axes, each of these is re- 

 pelled, with the same force, by the two poles. 



" The. force which causes this repulsion is independent of the 

 magnetism or diamagnetism of the mass of the crystal ; it de- 

 creases with the distance more slowly than the magnetic infuence 

 exerted by the poles f. 7 ' 



It is perhaps worth explaining, that if, on exciting the mag- 

 net, the optical axis take up the axial position, it is said to 

 be attracted; if the equatorial, it is said to be repelled. 



The first experiment of M. Pliicker, which led to the affir- 

 mation of these laws, was made with tourmaline. A plate of 

 the crystal which had been prepared for the purposes of po- 

 larization, twelve millimetres long, nine wide, and three thick, 

 was suspended by a strong fibre between the poles of an 

 electro-magnet. On sending a current round the latter, the 

 plate, which was magnetic, set itself as an ordinary magnetic 

 substance would do, with its longest dimension from pole to 

 pole. The optical axis of the crystal, thus suspended, was 

 vertical. 



On hanging the crystal, however, with its optical axis hori- 

 zontal; when the magnet was excited, the plate stood no longer 

 as a magnetic substance, but as a diamagnetic; its longest 

 dimension being at right angles to the line joining the poles. 

 The optical axis of the crystal was found to coincide with its 

 length, and the peculiar deportment was considered as a proof 

 that the optical axis was repelled. 



This law was farther established by experiments with Ice- 

 land spar, quartz, zircon, beryl, &c, and, as above stated, 

 included crystals of all kinds, both optic positive and negative. 



* Phil. Mag., vol. xxviii. p. 396. 



f Poggendorff's Annalen, vol. lxxii. p. 75- 



