its Circular Components in the Faraday "Effect" 473 



the fluid salt passed over into the flame to produce a brilliancy 

 comparable with the lime-light itself. Until this arrange- 

 ment was made, it was not possible to observe the splitting of 

 the image. All other light was carefully excluded, as non- 

 homogeneous light produced a spectrum which prevented 

 resolution. It is needless to say that the surfaces o£ the 

 prisms should be figured to within a fraction of a wave-length, 

 a difficult matter on account of the softness of the glass* 

 Oxidization of the surfaces is also unavoidable, due to 

 moisture, and they should be carefully protected and used 

 soon after polishing. These prisms were made originally 

 by Messrs. Franz Schmidt & Haensch, Berlin, and later 

 refigured and repolished by Mr. Petitdidier of Chicago, and 

 also repolished several times in my own laboratory. The 

 dimensions of the diagonal faces were 50 x 100 millim.; 

 only about 65 millim. of the vertical height was used, the 

 vertical aperture being about 10 or 11 millim. Unusual 

 facilities for producing an intense and extensive field are also 

 necessary. In this experiment the air-gap was 53 millim., so 

 that the poles could not come in contact with the prism. The 

 terminal pole-faces were 100 millim. square, and reduced 

 from a circular cross section of 203 millim, diameter by 

 specially curved pole-pieces, to reduce leakage. The magnet 

 was one of the most powerful in existence, and weighed 

 about two tons. A description of it is given in the Phil. 

 Mag. of October 1897. About 100,000 ampere-turns were 

 used. The intensity of the field was measured both by a 

 bismuth spiral and by the rotation in CS 2 , and gave 8700 

 c.G.S. units approximately for this number of ampere-turns. 

 The magnifying powers used were 16 and 60. The best 

 definition was obtained with the lower power. By shifting 

 the prism the images of the double slit could be brought into 

 the field of view ; one or more images, corresponding to the 

 different orders of passages within the prism, being visible at 

 times, simultaneously. These were generally much blurred 

 and indistinct, but certain positions were obtainable where the 

 two images of the double slit could be definitely resolved. 

 The effective horizontal aperture was then about 20 millim. 

 When the circuit was closed the original double image | 

 became first blurred, and widened, and finally, when the field 

 had become constant, changed into a triplet thus | | |, the 

 central band being apparently double the intensitv of either 

 of the outer bands and midway between the original elements. 

 The angle subtended by the doublet was *0005, as stated above, 

 while the angle of separation calculated above gave the same. 

 The predicted change hence agreed with the observed effect. 

 Phil. Mag. S. 6. Vol. 1. No. 4. April 1901. 2 I 



