808 



Dr. Norman Campbell on 



from A emerge from the box (probably in some cases after 

 striking the walls of A again) through the wire gauze at the 

 bottom and fall on the electrode E, which is covered with soot 

 to prevent reflexion. Between E and A a potential difference 

 V 2 can be established, which is counted positive when A is 

 positive. The potential of a lead accumulator is taken as 

 2 volts. The current flowing to E was measured by a 

 compensation method, so as to make the results independent 

 of the intensity of the light falling on Z. It was found that 

 a small negative current flowed from E, even when V 7 was 

 less than — 2 and no electrons left Z ; this current was in- 

 dependent of the value of Y ly so long as Y 1 was less than — 2. 

 It doubtless represents the photoelectric action of scattered 

 light on the parts of the apparatus outside A ; the values of 

 the current given are in all cases those obtained by subtracting 

 the value obtained when V a = —2 from that obtained when V x 

 has the value under consideration. Table II. gives the 

 values of the negative current flowing to E for various values 

 of Vi and Y 2 . The number of electrons received by E was 

 never greater than 6 per cent, of those liberated from Z. 



Table II. 



v x = 



v 2 =o 

 1 



2 



3 



4 



6 



8 



10 



12 



14 



16 



20 



30 



076 



1-42 

 1-09 

 0-56 

 0-30 

 004 



1-50 

 1-28 

 1-05 

 0-78 

 0-47 

 0-03 



1-37 

 118 

 100 

 0-82 

 065 

 0-29 

 0-02 



10 



1-19 

 105 

 0-91 



0-68 

 047 

 021 

 0-02 



12 



1-06 

 086 

 0-74 



0*55 

 0-41 

 0-28 

 013 

 002 



16 



1-00 

 076 

 0-64 



: 46 

 034 

 0-26 

 0-18 



0-06 



20 



TOO 

 0-76 

 0-66 



: 46 

 0-34 

 0-25 

 0-18 

 013 



0*06 



30 40 



0-84 

 0-64 

 0-55 



0-39 

 0-28 

 0-21 

 017 



o : ib 



0-07 

 002 



0-72 

 0-54 

 0-47 



0*34 

 024 

 0-18 

 0-16 



o'ib 



0-06 

 001 



The results obtained with this apparatus are not nearly so 

 «asy to interpret as those obtained with that used by Gehrts 

 or v. Baeyer, for the 8 and reflected rays in their passage 

 from A to the electrode pass through the electric field between 

 A and Z ; the proportion of rays of a given speed which 

 emerge from A is probably not that of the rays liberated with 

 that speed from A ; it would seem probable that a large 

 proportion of the slower rays would be driven by the field 

 back into A and would fail to emerge. Or, again, some o£ 



