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



breadth alike, the axis of the prism stood from pole to pole, 

 whether the conical points or flat faces were used. Shorten- 

 ing the axial direction a little more, and suspending the cry- 

 stal from its obtuse angle, the axis between the flat poles stood 

 equatorial, and, consequently, the longest dimension of the 

 crystal, axial ; between the points, owing to the repulsion of 

 the extreme ends, the length stood equatorial. Similar ex- 

 periments were made with ccelestine and topaz ; but all with 

 the same general result. 



"I had the advantage," says Mr. Faraday, "of verifying 

 Pliicker's results under his own personal tuition, in respect of 

 tourmaline, staurolite, red ferrocyanide of potassium, and 

 Iceland spar. Since then, and in reference to the present 

 inquiry, 1 have carefully examined calcareous spar, as being 

 that one of the bodies which was at the same time free from 

 magnetic action, and so simple in its crystalline relations as 

 to possess but one optic axis. 



" When a small rhomboid about 0*3 of an inch in its 

 greatest dimension is suspended with its optic axis horizontal 

 between the pointed poles of the electro-magnet, approximated 

 as closely as they can be to allow free motion ; the rhomboid 

 sets in the equatorial direction, and the optic axis coincides 

 with the magnetic axis ; but if the poles be separated to the 

 distance of a half or three-quarters of an inch, the rhomboid 

 turned through 90° and set with the optic axis in the equato- 

 rial direction, and the greatest length axial. In the first in- 

 stance the diamagnetic force overcame the optic axis force ; 

 in the second the optic axis force was the stronger of the two." 



The foregoing considerations will, we believe, render it 

 very clear that the introduction of this optic axis force is al- 

 together unnecessar}' ; the case being simply one of local re- 

 pulsion. Mr. Faraday himself found that the crystal between 

 the flat poles could never set its optic axis from pole to pole ; 

 between the points alone was the turning round of the crystal 

 possible. We have made the experiment. A fine large cry- 

 stal of Iceland spar, suspended between the near points, set 

 its optic axis from point to point; between the distant points 

 the axis stood equatorial. The crystal was then removed 

 from the magnetic field, placed in an agate mortar and 

 pounded to powder. The powder was dissolved in muriatic 

 acid. From the solution it was precipitated by carbonate of 

 ammonia. The precipitate thus obtained, as is well known, 

 is exactly of the same chemical constitution as the crystal. 

 This precipitate was mixed with gum water and squeezed in 

 one direction. From the mass thus squeezed a model of Ice- 

 land spar was made, the line of greatest compression through 



