﻿362 Prof. J. J. Thomson on the 



circuit consisted of a coil of wire with the ends connected to 

 the outside coatings of two Leyden jars, the inside coatings 

 of which were connected with the extremities of an induction- 

 coil: the secondary coil of the transformer had ahout 30 turns 

 for each turn of the primary coil. It was heavily insulated, 

 and both primary and secondary were immersed in an oil- 

 bath. This transformer easily gave sparks 7 or 8 inches long 

 in air, and when connected to the terminals of a discharge- 

 tube made of uranium-glass produced a very vivid phos- 

 phorescence. When the phosphorescence was produced in 

 this way, the images after reflexion in the rotating mirror 

 had one edge quite sharp and distinct, though the other edge 

 was indeterminate in consequence of the duration of the 

 phosphorescence. 



When the images of the two bright phosphorescent strips 

 were observed in the telescope, after reflexion from the 

 rapidly revolving mirror, their bright edges were seen to be 

 no longer in the same straight line : if the images came in 

 the field of view from the bottom and went out at the top, 

 then the sharp edge of the phosphorescent strip nearest the 

 electrode was lower than the edge of the other image ; if the 

 direction of rotation of the mirror was reversed so that the 

 images came in at the top of the field of view and disappeared 

 at the bottom, then the bright edge of the image of the 

 phosphorescent strip nearest the negative electrode was higher 

 than the bright edge of the image of the other strip. This 

 shows that the luminosity at the strip nearest the cathode 

 begins to be visible before that at the strip more remote ; and 

 that the retardation is sufficiently large to be detected by the 

 revolving mirror. This retardation might be explained, (1) by 

 supposing it due to the time taken by the cathode-rays to 

 traverse the distance between the phosphorescent patches; or 

 (2) we might suppose that, though the cathode-rays reached 

 the two phosphorescent patches almost simultaneously, it took 

 longer for the rays falling on the patch at the greater dis- 

 tance from the cathode to raise the patch to luminosity. In 

 other words, there may be an interval between the incidence 

 of the cathode-rays and the emission of the phosphorescent 

 light ; this interval being greater the further the phos- 

 phorescent patch is from the cathode. This latter sup- 

 position cannot, however, explain the displacement of the 

 images for the following reasons : — The sharpness and bright- 

 ness of the edge of the image show that the phosphorescence, 

 when once it is visible, must attain its maximum brilliancy in 

 a time very small compared with the time taken by the mirror 

 to rotate through an angle large enough to produce the 



