444 Mr Bevan, Reflection and Transmission of Light , etc. 
For sodium light 
p = 3 . 10 15 , 
F = 3 . 10 10 , 
and 
so that 
— = 3 . 10 17 , for negative particles, 
m & r 
</> = § 10 ~ 8 a approximately. 
To obtain therefore a change of phase of of a wave-length 
a must be § 10 6 in electrostatic units. 
If a condenser consisting of two metallic films deposited on 
the sides of a thin plate of glass or mica be used, and light be 
transmitted through one of them and reflected at the second, the 
conditions for a large surface density could be obtained — if the 
reflecting surface be charged negatively, the effect of the positive 
6 
charge on the first film could be neglected if — for the positive 
metal be small compared with the same quantity for the negative 
charge. Suppose the glass between the films 10 -4 cms. in thick- 
ness, P the difference of potential of the films. Then 
10*. PK 
47T 
sa}^ 5 . 10 3 P, 
so that for the change of phase of of a wave-length P must 
= 300 about, in electrostatic units. Or in volts P = 90,000. It 
seems probable that the condenser would break down under this 
difference of potential. 
The effect therefore on the light is of such a small order that 
with the present methods of detecting difference of phase it would 
probably escape detection. The above theory makes no pretence 
at representing accurately the state of things actually existing, but 
it may be regarded as sufficiently near the truth to deduce the 
order, at least, of the effect considered. 
