710 



Dr. F. W. Aston on a 



and the centre of the hyperbolic system. For the field 

 between plates of this form is such that its intensity varies 

 directly as the distance from this line. 



Again in fig. 3 a beam of such rays generated by the use 



Fio-. 3. 



of a point source and an annular slit, if passed between two 

 charged concentric spherical surfaces, will travel in great 

 circles and come to a focus on the axis of the system. 



Principle of the Spectrograph. 

 The above devices will give bright spectra of the ravs in 

 terms either of or — ; what is actually required is an 



arrangement which will give a spectrum in terms of — onlv 



and, at least over a small range, independent of v. This is 

 done in the present apparatus as diagrammatically indicated 



Fig. 4. 



in fig. 4. The rays after arriving at the cathode face pass 

 through two very narrow parallel slits of special construction 

 S! S 2 , and the resulting thin ribbon is spread out into an 



