3 i2 THE POPULAR SCIENCE MONTHLY. 



hollow barrel or case at the top of the machine, pierced with an open- 

 ing, within which, as seen in the figure, the object to be experimented 

 with is attached to a fixed stand. On the opposite side of the case 

 there is another opening in a corresponding position, not shown in 

 the figure. The outer case does not revolve, but within it there is a 

 pair of disks (Fig. 2) rigidly connected upon a spindle which is turned 

 by the machinery. Each of these disks has four openings, those of 

 the one being not opposite, but midway between those of the other. 

 Of course, then, when these disks are inside the case, it is impossible 

 to see through. The arrangement is then set up in the window of a 

 darkened room, so that one side is turned toward the sun, and the 



Fig. 2. 



Disks op Phosphoroscofe. 



other toward the observer; and, when the disks are turned, the object 

 is alternately exposed to the light from one side, and to the eye from 

 the other ; that is, it is seen in a moment after exposure to light, and 

 the duration of the moment can be determined by the rapidity of the 

 rotation. The object, therefore, if not phosphorescent, will never be 

 seen by the observer, as it is always in darkness, except when it is hid- 

 den by the intervening disk. But, if its phosphorescence lasts as long 

 as an eighth part of the time of one rotation, it will become visible in 

 the darkness. Suppose, now, that the disks are made to revolve a 

 hundred times in a second, and that the body observed is visible, it is 

 then proved that its phosphorescence lasts the one eight-hundredth 

 of a second, that being the time which elapses between its exposure 

 to the sun and its exposure to the eye. When examined in this way, 

 a very large number of bodies show traces of phosphorescence, al- 

 though in some cases it is found to last no longer than the ten-thou- 

 sandth part of a second. 



The question was thus opened whether phosphorescence is not a 

 general property of matter, and, to determine this, with the conditions 

 of its manifestation, a more thorough investigation of the subject was 

 needed. Prof. Rood proposed to undertake it, using, if possible, an 

 instantaneous source of illumination — the electric spark. But, in en- 



