PHYSICS. 473 



lene was used, having an index of 1.58. {ZeUschr. f. Inat.-Kunde, Feb- 

 ruary, 1884, IV, 42; Nature, XXIX, 514, March, 1884.) 



Bertraud constructs his prism by taking a prism of flint glass of index 

 1.058 and cutting it along a plane making an angle of 7(P 43' 8" with 

 the terminal faces. These new faces are polished and cemented with a 

 substance of index equal or superior to 1.G58, placing between these 

 surfaces a cleavage prism of calc spar suitably adjusted. A ray of light 

 falling on the prism enters it and is incident on the plate of calcite, 

 which divides it into two, polarized in planes at right angles to each 

 other. The ordinary ray, whose index is 1.658, continues its part in a 

 direct line; but the extraordinary ray, whose index is less, does not 

 enter the spar at all. in this way a polarizing prism is produced which 

 has a field of 44° 46' 20". This prism closely resembles that of Feuss- 

 ner above described. (C. JK., September, 1884, xcix, 538.) 



ELECTRICITY. 



1. Mafjnetism. 



The magnetic moment of a bundle of iron wires has been frequently 

 compared with that of an iron cylinder, but the influence of the diame- 

 ter of the wire seems not to have been studied. Bakmetiefi* has filled 

 the core of a cylindrical coil, 148""" long and 32.2"""' in interior diame- 

 ter, with iron wires of I"'"' , of 2""" and of 5"" diameter, and magnetized 

 them by more or less intense currents. The magnetic moment of the 

 bundle of the finest wires, as well as its specific magnetism, was notably 

 superior to that of the two others, especially for intense magnetizing 

 currents. But when, instead of filling all the space within the coil with 

 the wire, only an anmilar layer of wire was placed within it, the spe- 

 cific magnetism of the fine wire, for the same current, was many times 

 greater than that of the larger wires. (J. Soc. Phys. Ghim. Rttssej XV, 

 142; J. Phijs., October, 1884, II, iii, 463.) 



According to Auerbach, an iron cylinder free from magnetism re- 

 ceives from a magnetizing force a normal magnetic moment correspond- 

 ing to that force, and which is measured by the current induced during 

 its development. Bakmetieff now shows that this law does not apply 

 to iron already possessing remanent magnetisnr. His experiments 

 show that for electro-magnets not forming a closed magnetic circuit 

 the normal magnetism has for its measure the arithmetical mean of the 

 two induced currents obtained by closing the magnetizing circuit, di- 

 rectly and inversely. For a closed circular electro- magnet the normal 

 magnetism has double this value. {J. Soc.Fhys. Chim. Bi(sse, xv, 173; 

 J. Fhys., October, 1884, II, iii, 464.) 



Some time ago Quincke showed that electric force produces a press- 

 ure u\)ou insulating liquids placed in an electric field. He has now 

 extended his investigation to magnetic and diamagnetic fiuids placed 

 in a magnetic field. Calling .the diamagnetic constant for air unity, he 



