FACTORS OF A CORPUSCLE. 55 



light that causes this corpuscular migration is the ultra- 

 violet waves that is, waves that are too short to cause the 

 sensation of sight to the eye, and which, on dispersing the 

 light through a prism, lie beyond the violet end of the spec- 

 trum. These ultra-violet light waves, though they do not 

 affect the eye, are nevertheless for the most part the active 

 agents in affecting the sensitive plate of a camera, or in 

 tanning the skin. Sunlight is not rich in them, for the at- 

 mosphere absorbs them ; but a copious supply is found in 

 the light obtained from an arc lamp, or by burning magne- 

 sium wire, or by sparking with an induction coil between zinc 

 or cadmium terminals. 



Our task is to weigh and to determine the motion of and 

 the charge on the corpuscles as they fly off from a metal 

 plate under the incidence of ultra-violet light. This is 

 done by means of an apparatus shown in Fig. 12. A is 

 a charged aluminum plate on which the ultra-violet light 

 shines ; this light comes from a spark between zinc termi- 

 nals connected with an induction coil, and enters the tube 

 through the quartz window, B, which is peculiarly trans- 

 parent to ultra-violet waves. E is another metal elec- 

 trode, perforated in the middle. It shields the right-hand 

 part of the apparatus from the electrified plate, A, and 

 provides a window through which the corpuscles may fly 

 into the right-hand chamber. D and C are little metallic 

 plates that can be connected with an electrometer. 



All this apparatus is enclosed in a sealed glass vessel from 

 which the air has been almost altogether exhausted by pump- 

 ing. On sending the ultra-violet light through the quartz 

 window, B, so that it falls on the charged aluminum plate, 

 A, corpuscles fly off from A at right angles to it, and, pass- 

 ing in a beam through the window of the screen, E, they 

 strike the metal plate, D, give up their electricity to it and 



