Energy Distribution in Spectra. 51 



attained in a fraction of a second; so that the observer could 

 satisfy himself that the current was of the proper magnitude 

 during the brief space of six seconds allowed him. A thermo- 

 nmmeter requires several seconds to reach its final position, 

 so that for the purposes of this research an expensive 

 milliammeter would have been actually less satisfactory than 

 the simple arrangement here described. 



The Monochromator. 

 This instrument was designed by Mr. John Wylie, B.A., 

 of the Physics Department, and was constructed in the 

 laboratory workshop. Light focussed on the incidence-slit I 

 (fig. 1) was rendered parallel by reflexion from the concave 

 mirror M x and dispersed by a dense glass prism of 60° angle. 



Fiff. 2. 



The dispersed beam was reflected from the concave mirror 

 M 2 , and the spectrum focussed in the plane of the slit E. 

 Mi and the prism were rigidly mounted on a horizontal steel 

 arm which was pivoted below M l5 and could be moved by 

 the rotation of a screw S (fig. 2) attached to a drum D. 



E 2 



