74 Scientific Intelligence. 



two copper diaphragms, each perforated with one slit, were so 

 placed as to cause the long axes of the target and slits to lie in 

 the (geometrical) surface of a right circular cylinder. This 

 portion of the apparatus was supported inside by a double coil 

 of the Helmholtz type. Accordingly, when the magnetic field 

 was excited, the paths of the photo-electrons were bent around 

 in circles or helices some of which would have just the right 

 radii to pass through the two slits. After so doing, these elec- 

 trons entered a copper cup and indicated their arrival by the 

 deflection of the associated electrometer. i 



If r = radius of path and H = magnetic field strength, then 

 the speed of the photo-electrons may be calculated at once from 

 the relation 



v = — r M. 



in 



The impinging frequency corresponding to any particular speed 

 is determined by the equation 



y 2 m v 2 = e V = h(v ■ — v ). 



In this formula v denotes the frequency at which photo-electric- 

 emission from copper begins. Using m = 0-898 X 10~ 2T , h — 

 6-55 X lO" 27 , and v = 0-38 X 10 15 it follows that 



v = 00686 v°- + 0-38 X 10 15 . 



Therefore, the maximum and minimum frequencies can be com- 

 puted directly from the corresponding speeds. 



The most important results obtained from this investigation 

 may be summarized as follows: "The helium spectrum extends 

 to a limit which is certainly between 470 and 420 and probably 

 near to the latter value. The hydrogen spectrum terminates at 

 a wave-length between 830 and 950 Angstrom units and probably 

 close to 900. The mercury spectrum terminates at a wave-length 

 between 1000 and 1200 Angstrom units. 



So far as we are able to ascertain, the observed terminal fre- 

 quencies are identical with the frequencies calculated from 

 Bohr's theoretical values of the ionization potentials of the 

 respective gases, and they exhibit no obvious relationship to the 

 ionization potentials which have been determined experimentally 

 by Franck and Hertz and others." 



"The high frequency limits of the spectra referred to are 

 independent of the applied potential up to about 800 volts, pro- 

 vided this potential exceeds a lower limit which has not been 

 determined with any accuracy." — Phil. Mag., 34, 285, 1917. 



h. s. u. 



7. Navigation; by Harold Jacoby. Pp. xi, 330. New 

 York, 1917 (The Maemillan Co.). — In his preface, the author 

 sets forth very clearly the definite aims which he has endeavored 

 to attain in preparing the text. These may be summarized as : 



