Investigation of Polarized Light. 225 



incidence, therefore, excites a photo-electric current stronger 

 in the ratio x : y or A : B than a ray of equal intensity 

 vibrating in a plane at right angles to the plane of incidence, 

 and therefore parallel to the surface of the kathode. 



It is easily seen that the ratio A : B must depend upon the 

 angle of incidence. For with normal incidence the position 

 of the plane of incidence is undetermined, and therefore the 

 difference between A and B must disappear. Experiment 

 shows that the common value of the constants for this 

 direction of the rays is comparatively small ; whilst then with 

 increasing angle of incidence A increases rapidly, attaining 

 a maximum at about 60° and then decreasing, B becomes 

 continually smaller and appears to become zero at a nearly 

 grazing incidence. Thus between 60° and 70° the ratio A : B 

 has the value of about 50 : 1 (cf. the tables). Experiments, 

 still in progress, make it not improbable that the angle of 

 incidence for which A attains its maximum coincides with 

 the angle of polarization for the potassium- sodium alloy for 

 the most electrically active rays, that is, for the blue rays. 



It is an obvious suggestion that the different sensitiveness 

 of the metallic kathode-surface to light polarized in and at 

 right angles to the plane of incidence is connected with the 

 greater depth to which, according to Quincke*, the latter 

 penetrates into a metallic surface. 



We have then the remarkable result that a ray of polarized 

 light exerts a much smaller photo-electric effect at normal 

 incidence than if it strikes the kathode at an acute angle with 

 the plane of polarization at right angles to the plane of inci- 

 dence. But it is to be observed that the like result must 

 follow also with ordinary light, since we may regard this as 

 consisting of two components polarized in planes at right 

 angles, of which one vibrates in the plane of incidence. The 

 one at right angles contributes little to the photo-electric effect 

 at high angles of incidence on account of the smallness of the 

 constant B. 



In order that this prediction should be verified it is neces- 

 sary that the surface of the kathode should be such a perfect 

 plane as is only attainable by the use of the alkaline metals in 

 the fluid condition. Kathodes of solid sodium or potassium 

 have always a rough crystalline surface, and offer to the light 

 elements of all possible positions. Consequently the increase 

 of the photo-electric current with increasing angle of incidence 

 can only be observed with cells containing fluid kathodes of 

 NaK-alloy, whilst for those with solid kathodes the strength 

 of the current is almost independent of the angle of inci- 

 dence. 



* Quincke, Pogg. Ann, cxxix. p. 117 (1866). 



