MICHELSON. — THE ECHELON SPECTROSCOPE. 



119 



Figure 7. 



The second process differs from the first only in that each plate is 

 cut independently to the necessary height to give the required retardation. 

 The first approximation being made, 

 the plates are placed on a plane surface, 

 as in Figure 7. 



The projections a and h (Fig. 8) are 

 then ground and polished until the up- 

 per surfaces are all parallel, and the 

 successive retardations equal. The 

 parallelism as well as the height is 

 verified by means of the interferom- 

 eter. 



These processes are, it is freely 

 conceded, rather delicate, but prelim- 

 inary experiments have shown that with patience they may be suc- 

 cessful. 



Figure 8. 



