12 REPORT 1855. 



The form of the crystals belongs to the oblique system. The apparent longi- 

 tudinal axis of the broad six-sided prisms makes an angle of nearly 100° with the 

 base. There is a most perfect cleavage parallel to the axis in only one direction, 

 which bisects the prism of 135° 39'- The plane of the optic axis is perpendicular to 

 this plane of cleavage. One of the axes of elasticity makes with the plane of cleavage 

 an angle of about 10°. If now the crystals are examined as to their polarization in 

 a direction perpendicular to the plane of the optic axes, it will be found that the 

 pencil polarized parallel to the above-mentioned axis, which makes the angle of 10° 

 with the faces of cleavage, freely passes the crystal, but that the pencil polarized 

 perpendicularly to it does not pass. It is true, there appears not exactly a black 

 tint, but only a more or less dark gray ; but the contrast nevertheless is very striking. 

 On the mode of examination being reversed, the effect is still more powerful. A 

 plate of cadmacetite cut perpendicular to the plane of cleavage, parallel to the axis of 

 the crystals, when held near the eye, will extinguish one of the two images of a 

 doubly refracting prism entirely, without letting pass a trace of light, if the plate be 

 only so much as one-fourth of an inch in thickness. 



It is the more unexpected to find such great contrasts in the modifying power of 

 these crystals in respect to light, as for the rest they are perfectly colourless. M, 

 Charles von Hauer has succeeded in obtaining crystals 3 inches long and 1 inch 

 thick, but they are always very little homogeneous, consisting of concentric 

 funnel-shaped portions, which makes it very difficult to extract larger portions fit 

 for being turned to advantage as a polarizing apparatus. It is deserving of notice, 

 that some particular very compact portions of the crystals do not possess that cha- 

 racteristic absorbing property. 



On the Optical Illusions of the Atmospheric Lens. 

 By Evan Hopkins, C.E., F.G.S. 



An Account of some Experiments ivith a large Electro-Magnet, 

 By J. P. Joule, F.R.S. 

 Prof. W. Thomson, in Mr. Joule's absence, brought the subject before the 

 Section. The relation of the exciting force to the sustaining power of a magnet was 

 the subject which it was the author's desire to examine, the laws arrived at being 

 very divergent from those usually received. The soft iron made use of in this magnet 

 was of such a nature, that, after magnetization by moderate currents, it always — 

 probably on account of intense magnetization on some former occasion — retained 

 a residual polarity which was always in the same direction. The magnet might 

 be excited by a current which developed a polarity opposed to the residual one ; 

 but on the interruption of the current, the latter re-appeared. With high power, 

 the lifting power fell short of being proportional to the square of the current ; but 

 with feeble excitation, Mr. Joule found the sustaining force to vary nearly as the 

 fourth power of the current strength employed. 



Photographs of the Hartwell Observatory, and of the Craig Telescope at Wands- 

 worth, were exhibited and described by Dr. Lee. 



On New Forms of Microscope, adapted for Physiological Demonstration. 

 By M. Nachot. 



Elucidations, by Facts and Experiments, of the Magnetism of Iron Ships 



and its Changes. By William Scoresb"V, T).D., F.R.S.S. Land. 



<^ Edin., Corresp. Mem. of Institute of France, &-c. Sfc. 



"The author first recapitulated, as the basis of his present communication, the 



theoretic prmciples — concerning the magnetism of iron ships and its changes, with 



the effects on the action of the compasses — which he had formerly brought before 



the British Association, and described more elaborately in his " Magnetical Inves- 



