492 Scienttfic Proceedings, Boyal Diihlin Society. 



foniid that tlie action of rlie ligbt wns on tlie cathode, and Hallwachs made 

 the important discovery that light could discliarge a negatively electrified 

 body, but not a positively electrified one. The discharge was cliiefly due to 

 the ultra-violet waves. 



Hallwachs' experiment is an easy one to repeat. It is of fundamental 

 importance to photo-electric science. A convenient arrangement is to 

 connecf a polished plate of zinc to a gold-leaf electroscope and use an arc 

 liglit to radiate to it. If we charge the gold-leaf and zinc plate negatively 

 and then turn the light on tlie zinc, the leaf rapidly collapses. If we charge 

 tlie system positively, we get no effect from the light. The insulation — best 

 of sulphur — must be carefully attended to, and no bright light allowed to 

 fall upon it. 



We can detect the photo-electric effect without first electrifying the 

 illuminated plate. A carefully insulated plate, brightly lit from a source 

 rich in ultra-violet rays, will acquire a positive charge. This charge will rise 

 to a certain potential dependent on the cliaracter of the light, and will rise 

 no more. 



These results find explanation in the emission of negatively electrified 

 particles from the illuminated plate. If the plate is negatively electrified to 

 start with, the expulsion of the particles is facilitated. If it is positive, they 

 are attracted back and tiiere is no effect. If the plate is neutral to start with, 

 the loss of negative electrons gives it a positive potential. This increases till 

 the attraction is such as to keep back the negative particles. Beyond this it 

 cannot increase. 



Since the date of these pioneer experiments much lias been ascertained 

 regarding the nature of these negatively electrified particles and the condi- 

 tions under which they are liberated. In this resume we cannot, of course, 

 enter into details. The leading facts may be stated as follows : — 



(1) The particles expelled carry each a negative charge of the unit 

 amount — in other words, the Faraday unit which appears in electrolysis 

 and in the determination of the valency of the elements. 



(2) The particles are alike in mass, no matter from what elemental 

 substance they may be evoked. This mass is very mucli less than that of a 

 hydrogen atom — about the thousandth part. They are identical with the 

 " electron," with the " corpuscle," with the " jS ray," and with the 

 " cathode ray." 



(3) The number liberated by the action of light in unit time is directly 

 proportional to the intensity of illumination. 



(4) The energy with which they are discharged is independent of the 

 intensity of the light, and is directly proportional to the frequency — i.e., 



