1894.] Images following Visual Impressions. 141 



centres, opposite open sectors might be obtained, of any aperture 

 from to 90. The apparatus was set up opposite a box containing 

 a 32-candle power incandescent lamp, with a variable resistance in 

 the circuit, the side of the box between the lamp and the disks being 

 covered with a sheet of ground glass. 



The sectors being in the first place opened as widely as possible, 

 I fixed my eye upon the centre of the double disk, and at once saw 

 Charpen tier's dark band upon the illuminated, background. 



The sectors were then gradually closed up, until the posterior edge 

 of the dark band approximately coincided with that of the sector.* 

 When this was accomplished it was found that the arc of the open 

 sector was equal to about -Jy part of the whole circumference. The 

 dark reaction, therefore, ceased in ( T \ of 1^ sees. =) -^ sec - after the 

 first impact of the light upon the eye. 



For more readily demonstrating the succeeding phenomena, it was 

 found convenient to again open the sectors a little, so that they 

 covered an angle of about 10 or 12. Resuming the observation, it 

 was seen that the posterior edge of the open sector was bordered by 

 a luminous fringe due to persistence. A little beyond the termina- 

 tion of the fringe there appeared an intensely black radial band, 

 estimated to cover a space of from 3 to 4, and easily visible even 

 upon the black ground of the metal disk, though it is shown far 

 more conspicuously upon a translucent disk made of stout writing- 

 paper, with a sector cut out. Lastly, after another interval of, 

 perhaps, 35 or 40, came the luminous recurrent image,f which, 

 with the yellowish light of the incandescent lamp, appeared to be of 

 a blue colour. By varying the angular aperture of the sector, it 

 was ascertained that the recurrent image appeared at a fixed interval 

 after the light was cut off, and not after its first impact. 



This method of observation revealed one other point of interest, 

 which seems hitherto to have escaped notice, though it is evident 

 enough with a Charpentier disk, when once attention has been 

 directed to it. The average illumination of the bright band inter- 

 vening between the dark band and the leading edge of the sector is 

 much more intense than that of the other portion of the sector. 

 Moreover, it is not uniform, but increases, gradually at first, and 

 very rapidly at last, from the leading edge up to the dark bnnd. 

 In fact when the light used is not strong, the luminous margin of the 

 bright band is a far more conspicuous object than the dark band 

 itself : it appears to glow almost like a white-hot wire. 



Charpentier states that, under favourable conditions, he has been 



* This was not a very easy operation, because the luminous sector was slightly 

 widened by persistence, especially near the circumference. 



f This, of course, cannot be seen upon a translucent paper disk being over- 

 powered by the transmitted light. 



