Effect caused by Incident and Emergent Light. 337 



in thickness the emergent beam gradually becomes more 

 predominant in its effect, until a certain thickness is reached 

 where the ratio of: emergent to incident light attains a constant 

 value 1*12 to 1*0. The ratio remaining constant over so 

 large a range of thicknesses for the thin films of metal, shows 

 conclusively that the absorption of light and electrons by 

 these films as the beam of energy passes through them is 

 negligible, and falls within the experimental error. So that 

 the intensity of the incident and emergent beams must be 

 sensibly equal, and hence the value arrived at above must 

 be a true difference between the incident and emergent 

 light. 



Relative Absorption of Quartz Plate. 



In order to determine how much of the emergent beam 

 was absorbed by the 1 mm. quartz plates, upon which the 

 films were mounted, the following method was resorted to. 

 A relatively thick film was sputtered on a 1 mm. quartz 

 plate. A thick film was preferably used because less error 

 was involved in the measurement of its photoelectric effect, 

 since for equal intensities of light relatively thicker films 

 gave a larger photoelectric effect per unit time of exposure 

 to the light. With this film the experiments for Position (1) 

 and Position (2) in fig. 2 were repeated, and its ratio of 

 emergent tD incident effect noted. 



A second blank quartz plate 1 mm. thick was now rigidly 

 fastened to the blank side of the sputtered plate B. So that 

 now the light had to pass through 2 mm. of quartz when the 

 above readings were repeated. This was repeated for 3 and 

 4 mm. quartz plates by addition of a 2 and 3 mm. plate to 

 the original 1 mm. sputtered plate B. 



The results were then plotted as shown in fig. 4 (p. 338) . It 

 is seen that for a value of emergent to incident effect for the 

 1 mm. plate equal to 1'10, a value 1'15 should have been 

 attained if no quartz had been present. So that plotting the 

 curve for 1 mm. quartz equal to 1*12 for very thin films, 

 parallel to the original curve, we get a value for the ratio of 

 the emergent to the incident beam equal to the ratio 1*17 to 

 1*0, or an increase of 17 per cent. 



The above results have been confirmed by reversing the 

 direction of the light. For this purpose a speculum metal 

 mirror was placed so that the reflected beam of light could 

 be alternately sent through the cylinders in opposite direc- 

 tions without changing the position of the plates. Experi- 

 ments were also made to determine whether a slight deviation 



