Determination of Refractive Indices 

 Plate VI— Olive Oil. w=267. Temp. 26°. 



15 



One of the difficulties met with in this method is the shifting 

 of the bands during the photographing, as occurs in all inter- 

 ference work. This was mechanical, and was overcome by shift- 

 ing the cell, for all the shift of the bands with respect to the 

 Fraunhofer lines originated in the cell itself. That the Fraun- 

 hofer lines and interference bands appear simultaneously is one 

 of the important features of this method. No matter what ap- 

 paratus is used, the scale of the Fraunhofer lines will always be 

 the same from which the measurements are made, and no me- 

 chanical errors can enter. The shift of bands which might arise 

 due to changes of temperature must be guarded against in all 

 methods. 



It is also possible with the normal spectrum to determine di- 

 rectly the index for any wave-length. In the prism method, we 

 are dependent upon the more prominent Fraunhofer lines for 

 points of reference. 



143 



