400 Mr. A. LI. Hughes on the 



e> 



X 1450 and X 1780. The greatest Telocity o£ electrons pro- 

 duced by the light from the arc corresponds to 3*0 or 3*1 

 volts, which means that the shortest appreciable wave-length 

 in the spectrum of the mercury arc is X 1230. 



It is perhaps necessary to justify the application of 

 Ladenburg's law to this investigation. From considerations 

 of Planck's theory of radiation, one would expect the 

 frequency of the light, and the velocity of the electrons to be 

 connected linearly. Ladenburg expressed his results in the 

 form \/ V\ = const. ; but Joffe * has shown that if Ladenburg's 

 results are plotted, the curve showing the relation between 

 the frequency and the velocity is a straight line which, 

 however, does not pass through the origin. As the linear 

 relation has the better support in theory, we may regard 

 Ladenburg's law \/YX = const, as an empirical relation 

 which is satisfied over a limited range. The work of 

 Ladenburg and Hull shows that this relation is true over 

 the range \ 1230 to X2600. Since the wave-lengths dealt 

 with are found to be within the above limits, the application 

 of Ladenburg's law to this investigation is justified. The 

 only substance available whose transparency was known was 

 thin quartz, and this sufficed to determine the constant in the 

 equation \/YX — const v while in Joffe's form there are two 

 constants to determine. 



The experiments of Professor Lyman show that the 

 spectrum of hydrogen extends as far as X 1030. He suggests, 

 however, that perhaps still shorter wave-lengths may be 

 emitted, but that either the photographic plates used in the 

 experiments are not sensitive to shorter wave-lengths, or 

 that the grating used for giving the spectrum does not act 

 efficiently beyond about X 1000. The electrical method of 

 detecting the shortest wave-length has the advantage that 

 the shorter the wave-length, the more sensitive is the test. 

 The experiments described above show that the mercury 

 spectrum ends at about X1230, while Lyman's work shows 

 that the hydrogen spectrum extends further, viz. to A, 1030. 



2. 



The following is a short account of an experiment on the 

 nature of the photoelectric effect. Several views as to the 

 mechanism of the effect may be considered. 



1. Light may be regarded as molecular in structure and 



Joffe ; Aim, der Tftys, xxiv. 5, p r 039 (1907). 



