420 



NA TURE 



IMarch 3, 1887 



amined in the spectroscope, not all the bands appear at the 

 same speed of rotation. At a slow speed the double greenish- 

 blue band of G3 (545) first comes into view, closely followed by 

 the deep blue band of Go (482). This is followed, on increasing 

 the speed, by the bright citron band of 05 (574), and at the 

 highest speed the red band of Gf {619) is with difficulty seen. 

 The followincr are measurements of the time of duration of the 



phosphorescences of the different constituents of yttrium. The 

 wheel was first rotated slowly, until the fisst line visible in the 

 spectroscope attached to the phosphoroscope appeared ; the 

 speed was counted, and it was then increased until the line next 

 visible was seen. In this way the minimum speed of revolution 

 necessary to bring each line into view was obtained, and from 

 these data the duration of phosphorescence for each constituent 



I'lG. I, B. 



•af yttria was calculated. The time in the following table re- 

 presents in decimals of a second the time elapsing between the 

 cessation of th; induction discharge and the visibility of the 

 residual glow of the earth : — 



At o'oo.'S sec, interval the 'green and blue lines of Gi3 and 

 Go begin to b^ visible. 



At 0'0032 sec. interv.il the citron line of G5 begins to be 



visible. 

 At 0-00175 ,, th3 deep rel line of Gf (647) is just 



visible. 

 AtO'ODi:5 ,, the line of G5 is almost as bright as 



that of G3, and the red line of 



Gt) is visible. 



