414 Proceedings of Royal Society of Edinburgh. [sess. 
the oxidised copper alone. In this case the deflection went 
towards an ultra- violet-light-zero at about + 6 scale divisions per 
minute ; and both positive and negative charges were discharged, 
the negative much more quickly than the positive. 
§ 21. The ultra-violet light was now shone between the oxidised 
copper and the disinsulated tinfoil wall opposite to it, parallel to 
their surfaces so as to illuminate both. The difference between the 
metallic zero and the ultra- violet-light-zero was found to depend on 
the distance between the two surfaces. This will be seen from the 
following table : — 
Distance 
Time required 
Ultra-violet- 
between 
to come to 
Jan. 
28. 
light-zero. 
surfaces. 
steady reading. 
12.20 
p.m. 
1F;n j sc. divs. from ) 
\ metallic zero \ 
4-3 
cms. 
4 
mins. 
2.0 
33 
+ 134 
3 3 
3 3 
3*0 
33 
9 
3 3 
2.10 
3 3 
+ 121 
3 3 
3 3 
2*0 
3 3 
5 
3 3 
2.20 
3 3 
+ 102 
3 3 
3 3 
1*0 
5 3 
5 
3 9 
2.40 
3 5 
+ 86 
3 3 
3 3 
0-6 
3 3 
5 
33 
2.50 
3 3 
+ 169 
3 J 
3 3 
4-0 
3 5 
10 
33 
3.0 
3 3 
+ 161 
3 3 
3 3 
5-0 
3 3 
5 
3 3 
3.20 
3 3 
+ 199 
33 
3 3 
7-0 
3 3 
5 
3 3 
[Sensibility of electrometer 140 sc. divs. per volt.] 
The fact that in experiments (2) and (6) a longer time was 
required before a steady reading was obtained, probably depended 
on the way the light fell on the surface and on variations in in- 
tensity of the light. 
In this table we see that the steady electrometer reading (which 
we have called the ultra-violet-light-zero) is largely influenced by 
the distance between the plates, being greater the greater the dis- 
tance. This is a very remarkable result. It was first discovered 
by Righi, and very clearly described in papers of his to which we 
have referred. It may be contrasted with the non-difference of 
electrometer readings for different distances between the plates in a 
volta-zinc-copper and single fluid cell. 
§ 22. [Added February 6. — We have also made an exactly simi- 
lar series of experiments with Rontgen rays. The same insulated 
oxidised copper pdate was placed inside the same tinfoil box, and 
the Rontgen rays shone in between the two metals so as to shine 
on both. The following results were obtained with the oxidised 
copper at different distances : — 
