1903.] Heliv.m and Mercury in a Magnetic Field. 1 7 



Apparatus. 



Electromagnet. The apparatus consisted of a set composed of a large 

 electromagnet and an echelon spectroscope of twenty-six plates with 

 auxiliary. The magnet was specially made for the purpose of these 

 experiments and some others. The iron part (yoke, limbs and poles), 

 is rectangular in form, and is about three feet long by about a foot 

 and a quarter deep. The permeability of the cast-steel used for the 

 yoke and limbs coincides very nearly with that given by Hopkinson for 

 wrought iron : f or H = 35 C.G.S. the value of B is 15.000 C.G.S. A 

 large magnetising coil surrounds the yoke and extends over its whole 

 length. The poles are provided with smaller coils about a foot in 

 length, having respectively 1140 and 1170 turns of copper wire of 

 No. 9 standard wire gauge. The coils can stand easily about 16 amperes, 

 and for short periods 50 or 60 amperes. The field intensity between 

 the poles, as tested by the usual ballistic method, is exceedingly great ; 

 but unfortunately we have not hitherto been able to observe the 

 Zeeman effect in a field of much higher intensity than 10,000 C.G.S., as 

 in all the tubes with which we have worked the light becomes so faint 

 at the higher fields that it is not possible to obtain readings. 



The Echelon. The echelon, which is by Hilger, London, has twenty-six 

 plates ; the thickness of each is 10*23 mm. ; the width of step is 1 mm. 

 The difference in wave-length between two lines under observation is 

 given by 



where d^ is the angular displacement between the two lines A, , 



d<f> 2 that between two successive orders of spectrum, 8 the thickness of 

 one of the echelon plates, and /x the refractive index of the glass for 

 wave-length A.. We have always determined the ratio dfaldfa by 

 means of a micrometer eye-piece fitted to the observing telescope. 



The factor, which multiplies dfa/dfa, was obtained in the following 

 manner. The refractive index of the glass, /*, was given by the maker 

 of the instrument for the four lines C, D, F, G. For D it is 1-5746. 

 Cauchy's relation between refractive index and wave-length was 

 assumed, namely, 



and in this equation were substituted for /* and A their known values 

 for three of the principal wave-lengths, giving three simultaneous 

 equations to determine the three constants] A, B, C. It was verified 

 that these values of A, B, C gave to less than a unit in the fourth 

 place of decimals the values of p furnished by the makers for the four 

 VOL. LXXII. c 



