S6 



University Studies 



Vol. IV APRIL, 1904 No. 2 



I. — On the Determination of the Refractive Indices by Means of 

 Channeled Spectra 



BY S. R. WILLIAMS 



If the light reflected from two parallel optical surfaces be ex- 

 amined with a spectroscope of sufficient resolving power, the result- 

 ing spectrum will be traversed by dark bands corresponding to 

 those wave-lengths which interfere, the center of a dark band oc- 

 curring when 2fxD = 2lV\/2 and the medium between the surfaces 

 is of a less refractive index than that composing the optical surfaces. 

 For the center of a light band 2/j.D=(2jV-}-i)\/2 where D is the 

 linear thickness of the space intervening, A the wave-length, and N 

 some integral number. In case /x becomes equal to unity, as oc- 

 curs when we have a vacuum, the value of IV in the above equation 

 may be derived, giving the length of the path D in wave-lengths 

 of light. Using the sun as a source of light there will appear in 

 the spectrum the Fraunhofer lines, in addition to the interference 

 bands, thus furnishing a scale of reference for every interference 

 band. 



If two dark bands appear in determinate parts of the spectrum 

 corresponding to wave-lengths Ai and A2, then by the above equa- 

 tions 2ii.\D=N\\ for the longer wave-lengths, and 2fi-2D= 

 (JV-}-m)\.2 for the shorter, whence, assuming /x=i, we obtain 

 mX-2 



N=: 



Ai — A2 



University Studies, Vol. IV, No. 2, April, 1904. 



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