114 



Mr. W. Crookes. 



[Feb. 17, 



when the breaks will be much longer than the makes, or it can be set 

 to rub near the base of the teeth, when the current will remain on for 

 a much longer time and the intervals of no current will be very short. 

 By means of a screw, 1 1, attached to the spring, any desired ratio 

 between the makes and the breaks can be obtained. The intermittent 

 primary current is then carried to an induction coil, m, the secondary 

 current from which passes through the vacuum tube, n, containing- 

 the earth under examination. When the commutator, the coil-break, 

 and the position of the vacuum tube are in proper adjustment, no 

 light is seen when looked at from the front if the wheel is turned 

 slowly (supposing a substance like yttria is being examined), as the 

 current does not begin till the tube is obscured by an intercepting 

 segment, and it ends before the earth comes into view. When, how- 

 ever, the wheel is turned more quickly, the residual phosphorescence- 

 lasts long enough to bridge over the brief interval of time elapsing- 

 between the cessation of the spark and the entry of the earth into the 

 field of view, and the yttria is seen to glow with a faint light, which 

 becomes brighter as the speed of the wheel increases. 



To count the revolutions, a projecting stud, o, is fastened to the 

 rotating axis, and a piece of quill, p, is attached to the fixed support, 

 so that at every revolution a click is produced. With a chronograph 

 watch it is easy in this way to tell the time, to the tenth of a second,, 

 occupied in ten revolutions of the wheel. 



Under ordinary circumstances it is almost impossible to detect any 

 phosphorescence in an earth until the vacuum is so high that the line 

 spectrum of the residual gas begins to get faint ; otherwise the feeble 

 glow of the phosphorescence is drowned by the greater brightness of 

 the glowing gas. In this phosphoroscope, however, the light of 

 glowing gas does not last an appreciable time, whilst that from the 

 phosphorescent earth endures long enough for it to be caught in the 

 instrument. By this means, therefore, I have been able to see the 

 phosphorescence of yttria, for example, when the barometer gauge 

 was 5 or 6 mm. below the barometer. 



When the earth under examination in the phosphoroscope is yttria- 

 free from samaria, and the residual emitted light is examined in the 

 spectroscope, not all the bands appear at the same speed of rotation. 

 At a slow speed the double greenish-blue band of G/3 (545) first 

 comes into view, closely followed by the deep blue band of Gx (482). 

 This is followed, on increasing the speed, by the bright citron band 

 of (574), and at the highest speed the red band of (619) is 

 with difficulty seen. 



The following are measurements of the time of duration of the 

 phosphorescences of the different constituents of yttrium. The wheel 

 was first rotated slowly, until the first line visible in the spectroscope 

 attached to the phosphoroscope appeared ; the speed was counted, 



