224 MM. Elster and Geitel on the Electro- optical 



the cell with parallel glass windows confirms that found with 

 the bulb-shaped ones ; and even when the ray directed 

 towards the centre of the kathode-surface cuts the glass wall 

 at an acute angle, as was the case with the cell referred to in 

 Table II., the regularity in the change in value of J is the 

 same. The reason of this is to be found in the fact that the 

 changes in intensity with change of azimuth, which a polarized 

 ray suffers at acute passage through a single surface of glass, 

 are only small even if the angle of incidence differs much 

 from the polarizing angle. 



For angles of incidence less than 40° (Table I. D) as 

 already remarked, the ray was reflected by a silver mirror 

 into the Nicol's prism. Strictly, we ought to take into 

 account the amount of elliptical polarization due to the 

 reflexion of the light from the silver, which itself would 

 cause a change in the intensity of the light transmitted by 

 the Nicol at different azimuths. But here also the error lies 

 quite within the limits of accuracy of the measurements. Of 

 this we convinced ourselves by measuring the brightness of 

 the emergent beam with a sodium cell with solid kathode at 

 right angles to the ray, during rotation of the Mcol through 

 90° : it remained almost constant. 



In order to eliminate the effect of possible change in the 

 zircon-light, the measurement with the first position of the 

 Nicol was repeated after each series of measurements. Only 

 those series were retained in which this control-measurement 

 agreed with the first. 



The relationship expressed in the above formula between 

 the photo-electric current and the azimuth of the light may be 

 deduced from the assumption, justified by previous experi- 

 ments, that the current-strength is proportional to the 

 intensity of the light, if we make the further assumption that 

 the constant is not the same for light polarized at right angles 

 to the plane of incidence as for light polarized in the plane of 

 incidence. If a denote the amplitude of a polarized ray, 

 whose plane of vibration makes an angle a with the plane of 

 incidence, then the intensities of its components parallel and 

 at right angles to the plane of incidence are respectively 

 a 2 cos 2 a and a 2 sin 2 a. The strength of the photo-electric 

 current caused by this ray is therefore 



J = a 2 os cos 2 a + a 2 y sin 2 a, 



if we represent by x and y the two constants between the 



intensities of light and current. In this expression, a 2 x and 



. a 2 y are the constants, independent of a, which were denoted 



above by A and B. A ray of light vibrating in the plane of 



