136 BARUS— ELLIPTIC INTERFERENCE [April 21, 



vised apparatus admit. If this adjustment is not perfect A^'q changes 

 with c. From equation (12), moreover, 



(12') 



since A'^o is constant only relative to e when 6 varies. 



10. Deviation per Fringe, etc., dO/dn, dO/de. — These measure- 

 ments are still more difficult in the absence of special apparatus, 

 since e is not determinable and the counting of fine flickering fringes 

 is unsatisfactory ; but the order of results may be corroborated by 

 observing the numljer of fringes between two Fraunhofer lines, like 

 the C, D and other lines used. Differentiating equations (8) and 

 (10) for variable n, A, 6, and N (since dN/dO is equal to dN^/dO, 

 equation (12')) and inserting — D cos 6- dd/dn=dX/dn, it follows 

 after arranging that 



(13) 



dO X- I + dNjdn 



dn eD i + cos {i + ^) eD cos /(cos i + cos &) 



or 



dO X 1-6 



.tan 



dn e cos / 2 ' 



Combining this with (11) 



dd \ sin / — sin 6 



^ ' dn eD cos / e cos / 



Since, in equation (13), e is not determinable it is necessary to com- 

 pare increments Adn/dO in terms of the corresponding increments 

 A^, whence 



(15) A{dn/dO) = ( cos // X tan ^-^ j A^'. 



Table I. also contains data of this kind com[)uted separately for the 

 Fraunhofer D, C, etc., employed and their mean values. To find 

 the mean width of fringes between these lines, their angular devia- 

 tions were divided by the number of fringes counted between them 

 at different values of e. The results agree as closely as the difficulty 

 of the observations warrants. One mav note that without remov- 



