Determination of Refractive Indices 5 



accurately determined by means of the interferometer, and the 

 presence of a vapor does not affect it, as it has been ascertained 

 that the refractive index of dry air is only about one-millionth 

 greater than air saturated with a vapor. The values, as found 

 by various investigators, vary by little over one part in a million. 

 Air has also been used as a medium of reference by other inves- 

 tigators, and so is used in this work. 



The direction of variation in thickness of the cell is at right 

 angles to the interference bands, since along their length the 

 thickness must be uniform. They will be parallel to the Fraun- 

 hofer lines, then, when the refracting edge of the cell is parallel 

 to the image of the slit. This was the means used for getting 



the plates of the cell parallel in a vertical direction, for, if the 

 interference bands were brought into coincidence above and below 

 the dividing strip, before the liquid is introduced, the value of D 

 for both films must be the same. 



Another condition necessary to give D an equivalent path, in 

 both films, is that the incidence be normal upon their surfaces, 

 for, owing to refraction at the bounding surfaces, the ray will 

 be bent more or less for one than the other, depending upon the 

 difference in index, consequently D would vary for the two films. 

 In the system used, normal incidence was not possible, but it 

 was made so small that it entered no appreciable error. Let AO, 

 fig. 3, represent the path of normal incidence upon air film xy 

 contained between two plates. Let BOC be the path of a ray 

 at an incident angle of 0,-. Since the film xy is an air film, the 

 ansrle of refraction at its surface will be the same as the angle of 



133 



