346 Prof. D. B. Brace on Light Propagated in 



Sensibility. Aperture. 

 Jan. 25, 1894. 9 Amp 2/40 10 mm. No effect. 



„ 27, „ „ Poor Sun. 



„ 29, „ „ 



„ 30, „ „ 



Feb. 1, „ „ 



Sensibility. 



Aperture. 



2/40 



10 mm. 



•02° certain. 



„ 



•02° 



,, 



•01° certain. 



„ 



01° to -02° 2/40 



,, 



1/40 to 2/40 



4 mm. 



01° 1/40 certain. 



5 mm. 



1 /40 certain. 



8 mm. 



01° 



8 mm. 



1/40 certain. 



6 mm. 



„ 12, „ „ 



„ 13, „ „ 



„ 24, „ „ 



,. 26, „ 



Knowing the specific rotary power cp D of the liquid and the 

 intensity of the magnetic field, the rotation p by the latter 

 method may be calculated from Verdet's law. cf> was deter- 

 mined by wrapping some 25 turns of wire about the tube, 

 and correcting for the ends ("992) and observing the double 

 rotation (1° 24/) of the plane of polarization produced by a 

 reversal of the current (11*73 amp.), the ray being from 

 sodium light and traversing the tube once. 



Substituting in Verdet's equation, we have 



84' _ _1_ a , ir/ 



9d ~4,7tx 25x1-173 X -992 X *~ ii0 ' 



The intensity of the field H was measured with a spiral of 

 bismuth whose constant was known. With an exciting 

 current of 9 amp., H was found to be 2600 C.G.S. units. One 

 turn of the screw M corresponds to an angle whose sine is 

 •000446, since 1/40 x 1/32 x 1/70 = -0000 1 116, the length of 

 the arm A being 70 in. and the pitch 1/32 in. 



The total length of the liquid column was 196*64 cm., and 

 that of the glass caps of the tube 3*2 cm. = 1*4 cm. of liquid, 

 making a total equivalent length of 198 cm. Hence we have 



•115' 

 p = ^- x 198 x -00001116 x 2600 = *011°, 



the observed value being *01° by the first method of deter- 

 mining the sensibility. 



It is assumed that the transmitted light was homogeneous 

 and that of sodium, which it was not, but appeared a deep 

 orange-colour. 



Incidentally a few observations were made on Verdet's 

 law near the neutral plane. The arm A could be moved from 

 this plane by about 10 turns of the screw M. As homo- 

 geneous light could not be used, the results are only approxi- 

 mate for the constant, but should be correct for the cosine 



