TRANSACTIONS OP THE SECTIONS. 15 



To give an idea of the degree of accuracy of which the instrument is susceptible, 

 suppose the ratio of the axes of the ellipse described to be about 3 to 1. In this 

 case the author found that the mean error of single observations amounted to about 

 a quarter or the fifth part of a degree in the determination of the azimuth, three or 

 four thousandths in the determination of the ratio of the minor to the major axis, 

 and about the thousandth part of an undulation in the determination of the re- 

 tardation. 



On account of the accuracy with which the retardation is determined, and the 

 largeness of the chromatic variations to which it is subject, the instrument may be 

 considered as determining, not only the elements of the ellipse described, but also the 

 refrangibility of the light employed, or its length of wave, which corresponds to the 

 refrangibility. The author stated that the error of the thousandth part of an undu- 

 lation, to which the determination of the retardation was subject, corresponded to 

 an error of only the twentieth or thirtieth part of the interval between the fixed lines 

 T) and E of Fraunhofer. ^ 



On Diamagnetism and Magnecrystallic Action. By John Tyndall, Ph.D. 



One of the most important inquiries which at the present day occupies the atten- 

 tion of the student of physical science, is the relation which subsists between mag- 

 netism and diamagnetism. Are the laws which govern both forces identical ? Will 

 the mathematical expression of the attraction in the one case be converted into that 

 of the repulsion in the other case by a change of sign from positive to negative ? 



To this question, M. Pliicker replies " No." His experiments have led him to the 

 conclusion, that where the power of a magnet which operates upon a body com- 

 posed of magnetic and diamagnetic constituents is increased, the diamagnetism of 

 the compound mass increases in a much quicker ratio than the magnetism ; that in 

 consequence of this, an indifferent body is a physical impossibility ; for a body in 

 which the respective forces might be exactly equal and opposite, when excited by a 

 magnet of a certain strength, would, upon lowering the power of the magnet below 

 this standard, be attracted, and by increasing the power of the magnet beyond this 

 standard be repelled. 



During a previous investigation, the author of the present memoir had repeated 

 opportunities of observing phaenomena exactly similar to some of those which form 

 the premises of Pliicker's conclusion, and a close study of the subject convinced him, 

 that, to account for these phoenomena, the hypothesis of two conflicting forces in the 

 Bame compound mass, the one or the other of which predominates according as the 

 power of the magnet is increased or diminished, was by no means necessarjr. 



To fit himself for the investigation of this question, he commenced an inquiry last 

 November into electro-magnetic attractions ; one cf the results of this inquiry was, 

 that a sphere of soft iron separated from the end of a straight electro-magnet by a 

 small fixed distance, was attracted by the latter with a force exactly proportional to 

 the square of the exciting current. Now this attraction is jn each case the product 

 of two factors, one of which expresses the magnetism of the magnet, and the other 

 the magnetism of the ball, and it is easy to see that while the attraction increases as 

 the square of the current, the magnetism of the ball increases in the simple ratio of 

 the current itself. 



Our way to a comparison of magnetic attraction and diamagnetic repulsion is 

 now clear. We know the law according to which the magnetism of an iron ball 

 increases, and we have only to inquire whether the diamagnetism of a bismuth 

 ball follows the same law. The apparatus used in the former case proved, however, 

 to be totally unfit for the measurement of diamagnetic force, — the feebleness of the 

 latter rendered a much more delicate mode of measurement necessary. 



The torsion balance was the instrument finally resorted to by the author. A loop 

 of paper was attached to one end of a fine silver wire, and in the loop rested a little 

 beam of light wood. At the ends of the beam, which was 6 inches long, two spoon- 

 shaped hollows were worked out, in each of which a ball of the substance to be ex- 

 perimented with might be placed. Two cores of soft iron, surrounded by helices of 

 copper wire, were placed at right angles to the beam when horizontally suspended, 

 the one core facing the ball at one end, and the other core facing the ball at the 



