212 On a Simple Method of Reducing Prismatic Spectra, 



possible, the spectrum will be found to consist of bright 

 lines separated by almost black intervals. The best results 

 will of course be obtained when the plates are in such a 

 position that the slit is parallel to the direction of the inter- 

 ference-bands seen with sodium light. The closeness of the 

 bands will depend on the thickness of the air-film between 

 the silvered surfaces. For photographic purposes we have 

 adopted the plan of covering either the upper or lower half 

 of the slit with a piece of black paper stuck on with soft red 

 wax before placing the plates in position. The necessary 

 exposure will vary from about half a minute to three minutes 

 (using Edwards's snap-shot isochromatic plates) according to 

 the nature of the spectrometer employed. It is well to in- 

 troduce a little common salt into the arc while this exposure 

 is being made, as thus the D lines, as well as the H and K 

 lines, will be superimposed on the bands. Another piece of 

 black paper having been placed so as just to cover the ex- 

 posed half of the slit, the first piece is removed, and the 

 spectrum which it is wished to examine is photographed. 

 Fig. 1 is a specimen of an iron spectrum together with a re- 

 ference interference-spectrum obtained in this way. It will 

 be noticed that the interference-bands in the violet part of 

 the spectrum are slightly inclined to the vertical. It is easy 

 to adjust the glass plates so that this is not the case, but we 

 have selected this photograph in order to point out that if 

 readings are taken on the line of junction of the two spectra 

 no error will result from such a want of adjustment. 



Starting from the red end of the spectrum every fifth and 

 tenth band can be marked and the whole numbered. The 

 following exhibits the procedure when the wave-lengths of 

 only a few lines are required. 



One or other of the D lines and the H or the K line will 

 generally be found to form the best datum lines. In the 

 present case the following datum lines were used : — 



Scale No. 90-2. Wave-length 5328*5 (\ ) 



„ 402<3. „ 3968-6 (XJ(H). 



Then according to equation (1) 



m\ m 



n= 



m=402-3-90-2=312-l 5 



\ -X w = 1359-9; 

 hence Qin#Q 



