190o.] Helium and Mercury in a Magnetic Field. 19 



to be seen under a dispersion equal to that of a ray passing through 

 a 60 prism in the position of minimum deviation. Any required 

 part of the spectrum could be got into the middle of the field by a 

 rotation of the prism-table. The eye-piece of the observing telescope 

 of this auxiliary spectroscope was removed, and the collimator of the 

 echelon was placed coaxially with that telescope, and at such a 

 distance that the light rays, after passing through the object glass 

 of the telescope, were brought to a focus on the collimator slit of the 

 echelon. After emerging from the lens system of the collimator, 

 the rays passed through the echelon grating, and were viewed by 

 means of a telescope focussed for parallel light. This telescope was 

 provided with a micrometer eye-piece, whose indications, as already 

 stated, gave the ratio dfa/dfa. 



By means of a suitable arrangement the prism-table of the auxiliary 

 spectroscope could be rotated by the observer when taking readings 

 without moving away from his position. In a similar way the width 

 of the echelon slit could be adjusted. 



Results. The first observations were made at right angles to the 

 magnetic field on several of the helium lines, and on the green line of 

 mercury. The results were used to calculate the values of (/A./HA-, and 

 of e/m by the formula e/m = STTV rfA/HA 2 , where v is the velocity of light, 

 3 x 10 10 cms. per second. In every case the normal triplet was 

 obtained, and the separation between the extreme components found to 

 be proportional to H up to fields of 10,000 C.G.S., beyond which it 

 was difficult to obtain readings for the reason already stated. The 

 numbers obtained are stated in the following table, and in fig. 3 8X is 

 shown plotted against H for different values of the latter, for the red 

 line of helium. A similarly close proportionality was found in the 

 other cases : 



c 2 



