256 



Hull — Initial Velocities of the Electrons. 



fluorite plate. It was then adjusted in a vertical position so 

 that it could be raised enough for a quartz plate, l*5 mm thick, 

 to be placed between it and the cell C. Lyman* has shown 

 that quartz of this thickness absorbs all light below X 1450. 



The results with the quartz are given in Table IT and figure 

 4. The absolute values cannot be compared with those in Table 

 I, since the intensity of the light was not the same. The 

 scale of ordinates of the different curves in this, as in the pre- 

 vious figures, is so chosen that their maxima coincide. Curve 

 I, which gives the velocities without the quartz and with the 

 cell C evacuated, is the same as curve I in figure 2, and is 





Table U, 









S y Light 









Loss of Neg. Electricity by A. 







Coulombs per min. 



x 10- 9 





ot. of B 



l'5 mm 





l-5 mm 



volts 



Quartz screen 





Quartz + 1-5 C 



— 6 



— 0-88 





-0034 



-4 



— 0-79 





— 0-036 



— 3 



— 0-78 





— 0-031 



— 2-5 



— 0-285 





— 0-030 



— 2-0 



1-52 





— 0-029 



— 15 



5'55 





0-009 



— 1-0 



12-3 





0-223 



— 0-5 



21 -5 





0-845 







37-4 





1-86 



+ 0*5 







2-11 



+ 1-0 



40-3 





2-14 



+ 2-0 



42-1 





2-17 



+ 3'0 







2-18 



+ 4-0 



42-5 





2-20 



+ 6'0 



43-8 





2-17 



inserted here for comparison. It shows a maximum energy of 

 about 3-5 volts, produced by light of X 1230. Curve H, 

 obtained with the quartz, but no air in the screen cell, gives 

 a maximum energy of about 3 volts, which must be due to 

 light of X 1450-1500, the limit of transparency of quartz, f 

 Curve III shows the effect of both quartz and air screen. It 

 represents the same conditions as curve II in figure 2, except 

 that the small amount of light between X 1270 and X 1230 has 

 now been cut out by the quartz. The maximum energy is 

 now about 2 '4 volts, which must be due to light of X 1710, the 

 less refrangible limit of the air absorption band. 



* T. Lyman, Astrophys. J., xxv, p. 49, 1907. 



f Lyman's photographs show that the absorption of 2 mm of quartz is com- 

 plete below about I 1500, that of 0'2 mm below 1 1450. The limit for l-5 mm 

 will be between these values. 



