Mica Echelon Grating. 629 



spectrum, while the echelon easily resolved the Hg lines, 

 showing the effect of the introduction of retardation. 

 The constants of the grating were as follows : — 



t the Thickness of plates 0*05 mm. 

 Width of space 0'5 mm. 

 Retardation of each plate, 50 waves. 



Calculating the separation of the Hg lines by Michelson's 

 formula, we have 



mm. 



he 



_ t 



. SA 















where 



= 0-05 



M[ 



~A 



' A' 





t 

 A 



$A 

 A 



-94, 



= 280, 



Z6= 



=M 



94 

 280 = 



1 

 ~3 3 







or the distance between the Hg lines is J- of the distance 

 between the spectra. 



It will be found instructive to illuminate the slit-plate of 

 the spectrometer with a focussed continuous spectrum, and 

 observe the way the different orders of echelon spectra file 

 by when the continuous spectrum is moved across the slit, 

 showing the dependence of order on wave-length. 



If the slit be illuminated with white light and a continuous 

 spectrum be formed in the telescope by means of a prism, 

 this spectrum will be found to be crossed by heavy dark 

 bands when the echelon is placed in front of the prism. The 

 explanation of these bands makes a good problem for advanced 

 students. A clew may be found by repeating the experiment 

 with the slit-plate illuminated with the continuous spectrum 

 instead of white light, and moving this spectrum very slowly. 



It may be of interest to some to know that the Zeeman 

 effect can be shown with an echelon made of four interfero- 

 meter-plates, the light being the green rays from a mercury- 

 tube. 



Physical Laboratory of the 

 Uuiversity of Wisconsin, Madison. 



