MICHELSON. 



THE ECHELON SPECTROSCOPE. 



113 



that this quality of throwing an excess of light in a particular spectrum 

 is due to the character of the ruling diamond, which cannot be deter- 

 mined except by the unsatisfactory process of trial and error. 



If it were desired to produce rulings which should throw the greater 

 part of the incident light in a given spectrum, we should try to give the 

 rulings the form shown in section in Figure 1. 



/ 



Figure 1. 



I am aware of the difficulties to be encountered in the attempt to put 

 this idea into practical shape, and it may well be that they are in fact 

 insunnountahle ; but in any case it seems to be well worth the attempt. 



Meanwhile the idea suggested itself of avoiding the difficulty in the 

 following way. 



Figure 2. 



Plates of glass (pp, Fig. 2) accurately plane-parallel and of the same 

 thickness, are placed in contact, as shown in the figure. If the thicknesses 

 were exactly the same, and were it not for variations in the thickness 

 of the air-films between the plates, the retardations of the pencils 

 reflected by the successive surfaces would be exactly the same, the 

 reflected waves would be in the same conditions as in the case of a 

 reflecting grating, — except that the common retardation is enormously 

 greater. 



vol. xxxv. — 9 



