Method of Reducing Prismatic Spectra. 

 Fig. 2. 

 Diagrammatic Plan of Apparatus. 



215 



,SLlT 



KED, 



VIOLET 



SILVERED GLASS 

 PLATES 



Spectroscope. 



mities of the latter by a straight line. The frequency cor- 

 responding to any given scale-number is read off directly. 

 The accuracy attainable by this method, in which no calcula- 

 tions whatever are involved, is similar to that obtained by the 

 method previously described. 



Fig. 3. 



2500 



2400 



2300 



2200 



2100 



2000 



1900 















1 





fl/A39 































/ 















- 



/ 



/ 

















/ 

















/ 

















/ 















(Ca) 



A 5896 -5/1 00 



(Na)/ 



200 



300 



400 



It is, of course, unnecessary to take a separate photograph 

 of the reference interference-scale for every spectrum to be 

 examined. If the D lines are superposed on the original 

 interference-scale, and occur also in every succeeding spectral 

 photograph obtained, the reference scale can be photographed 

 once for all, provided the adjustments of the spectrometer 

 remain unaltered. The photographic scale can be placed with 

 its film in contact with that of the photograph bearing the 



