from a Zinc Plate by Ultra-Violet Light. 305 



It should be stated that other investigators have as a result 

 of their experiments asserted the constancy of the ratio E/I. 



Lenard (Ann. d. Phys. viii. 1902, p. 154) assumed that 

 the light-intensity for normal incidence varied as the inverse 

 square of the distance between the spark and the detector, 

 but did not apparently make any correction for the absorption 

 of the light by the intervening medium. As shown in the 

 present paper this for the larger distances will only reduce 

 the intensity of the light in a certain fixed ratio, so that the 

 following results extracted from Professor Lenard's paper 



will still hold good :- 





I. 



E. 



E-=-I. 



206 



236 



231 



977 



46-3 



4-64 



36-9 



794 



86-5 



1-44 



106 



7-42 



It will be observed that for a change of 1 : 22 in the light- 

 en . . ° 



intensity the corresponding variation in E/I is approximately 

 2b per cent. If in spite of this E/I is assumed to be constant 

 the degree of accuracy could not have been very high, and 

 probably the matter was not investigated with a suitable 

 apparatus. 



Ladenburg (Ann. d. Phys. 1903, p. 573) varies the angle 

 of incidence of the light and assumes that the light-intensity 

 is proportional to the cosine of this angle. When we consider 

 that the effective portion of the lightis that which is absorbed 

 by the zinc plate, and that this fraction varies with the angle 

 of incidence, the above assumption seems hardly justifiable. 

 But even if this is permissible, it should be observed that the 

 angle of incidence ranged in value from 0° to 80°, so that 

 the range of intensity was from 1 to 5'8. From 0° to 60 c 

 the intensity is only reduced by one-half, so that the greater 

 part of the variation took place in a limited region of 

 about 15°. 



It would be impossible, with our-present limited knowledge 

 of the w y ay in which the light acts on the molecules of the 

 gas, to form a theory which would indicate the connexion 

 between E and I. A little consideration, however, would 

 show that there is no particular reason for supposing that 

 the ratio between these two quantities is constant. 



Assuming that the ions are set free owing to the action of 

 the electric force (F) in the. beam of lighr. the experiments 



