Effect caused by Incident and Emergent Light. 855 



allowing for the absorption of the film and the 0'3 mm. 

 quartz plate on which the film was mounted. 



Had thinner films of platinum been used and allowance 

 been made for the absorption of the light by the quartz, I 

 find that his value would have increased from 1*15 to unity 

 to 1*17 to unity for the ratio of the emergent to the incident 

 photo-electric effect. He repeated his experiments in vacuo 

 and found that they checked his previous values when air at 

 atmospheric pressure was in his apparatus. 



In view of this work it was thought unnecessary to repeat 

 the above experiments in vacuo, since as they stand they are 

 confirmed by Kleeman's results. His measurements were 

 made by an electroscope method which is quite different from 

 the one used in this investigation. 



Some years ago Hallwachs * showed that a photo-electric 

 effect could be obtained with a silvered quartz plate when the 

 light was incident through the quartz. This he suggests can 

 be explained in two ways: either the light passes through 

 the silver and liberates electrons at the rear surface of the 

 metal, or the electrons acted on by the light at the incident 

 surface have velocities great enough to carry them out 

 through the rear side. A similar effect was found by Rubens 

 and Ladenburg f with gold-leaf. 



They found that the ratio of the photo-electric effect in 

 front and behind was 100 to 1*0, while the fraction of the 

 ultra-violet light transmitted, photo-electrically measured, 

 was one one-thousandth. That they did not and could not 

 have foreseen that the emergent photo-electric effect was 

 greater than the incident effect has been pointed out by 

 Kleeman J. 



The object of the present investigation is to determine the 

 magnitude of the ratio of the emergent to the incident photo- 

 electric effect for as many characteristically different metals 

 as practicable. In this connexion the phrase photo-electric 

 effect is to be understood as the total electronic emission due 

 to light of standard intensity. 



Method of Experimenting. 



The same apparatus and method of experimenting was 

 used as in the first paper on this subject §. A beam of ultra- 

 violet light from an iron arc was passed along the common 



* Hallwachs, Tagbl. d. Heid. Nat. Ver. S. 24 (1890). 



t Ladenburg-, Deutsch. Phys. Gesell. Verh. ix. p. 24, § 749-752 (1907). 



t Loc. cit. 



§ O. Stuhlmann, Jr., Phil. Mag. xx. p. 333, Aug-. 1910. 



3 L2 



