444 Mr Bevan, Reflexion and Transmission of Light, etc. 



For sodium light 



p = 3 . 10 15 , 

 F=3.10 10 , 



and — — 3 . 10 17 , for negative particles, 



so that cf) = | 10~ 8 (t approximately. 



To obtain therefore a change of phase of y^- of a wave-length 

 a must be flO 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 



e 

 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 4 PK 

 a = J ^' Sa ^ 5 - 1()3P ' 



so that for the change of phase of T ^ 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. 



