Prof. Knoblauch on the Deportment of Crystalline Bodies. 33 



cases under the same angle ; but in the second case we imagine 

 that it lies further away. Hence the idea of a larger surface 

 seen at a greater distance^ and the apparent diminution of the 

 inclination. 



This explanation is corroborated by the following experiment : 

 A small gypsum bust was placed before a polished hollow mirror, 

 so that the reversed image seen with both eyes fell beside the 

 bust. Without changing the position of the right eye, the left 

 eye was closed. The image receded immediately to the surface 

 of the mirror, and appeared much larger ; it was observed under 

 the same angle, but fancied to be at a greater distance. 



In comparative stereoscopic investigations the drawings must 

 always stand at equal distances. It is a proof that the combina- 

 tion is good if, when the head is moved slowly to and fro, the 

 relief is set in slow oscillatory motion. 



3. On the Deportment of Crystalline Bodies between the Electric 



Poles. 



It is well known that crystalline bodies suspended between 

 the poles of a magnet take up positions which distinguish them 

 from unciystalline. It will be of interest to examine whether a 

 similar deportment is exhibited between the electric poles. 



As in the case of bodies suspended between the poles of a 

 magnet, it is here absolutely necessary to annul all action which 

 might arise from a mere peculiarity of shape. This is done by 

 converting the substance into circular discs, which arc to be sus- 

 pended horizontally. 



A plate of heavy spar cloven parallel to the plane of most emi- 

 nent cleavage had the short diagonal of the rhombic base marked 

 upon it. Suspended between the poles of a dry pile consisting 

 of 400 pairs of zinc and gold-paper, or of 2000 pairs of silver- 

 paper and manganese, it set itself so that the short diagonal stood 

 perpendicular to the line joining the poles. 



For the sake of brevity, as in the case of experiments with the 

 magnet, we shall call this position the equatorial. 



A circular disc taken from a gypsum crystal parallel to the 

 plane of most eminent cleavage, set itself so that a line, which 

 slightly deviated from the short diagonal of the rhombus inclosed 

 by the other cleavages, stood equatorial. 



These experiments require the greatest caution ; only such 

 crystals should be used as are entirely free from all traces of 

 fracture. Seven examples coincided in the deportment just 

 described. The action is stronger in the following crystals : — 



Saltpetre was so cut that the crystallographic axis lay in the 

 plane of the disc; between the electric poles the axis stood 

 equatorial. 



Phil Mag. S. 4. Vol ». No, 8. July 1851. D 



