ADAMS: EAYLEIGH-ZEISS INTERFEROMETER 



271 



crown glass (n = 1.52) is 0.48. We shall have occasion to use 

 formula (10) when we come to the subject of the alteration of 

 the achromatic fringe in white light. 



As a check on the above relations some observations with 

 monochromatic light (obtained by means of a Monochromatic 

 Illuminator) of several wave lengths were made on the relation 

 between the number of divisions on the drum and the number 

 of fringes crossing the field. For wave lengths 0.5876 and 

 0.5461 He and Hg tubes respectively were used, and an arc lamp 

 for wave-lengths 0.6396 and 0.4861, the slits of the Illuminator 

 in the latter case being very narrow, in order to obtain sufficiently 

 homogeneous light. The character of the results is shown by 

 Table I for wave-length 0.5876 — . 



Average = 0.8 



The first and second columns give respectively the number 

 of fringes N and the corresponding observed number of scale 

 divisions r. The third column gives the values of r' calculated 

 by equation (6 a) and the fourth, the quotient r'/N' which in- 

 spection shows to be practically constant, as it should be. The 

 least square mean value of this quotient is 22.447; the differences 

 between this mean and the individual observations, when com- 

 puted back in terms of r, are given in the last column, which 

 shows that this discrepancy is no larger than the error of reading 

 (1 division, which is less than 1/20 of a band). 



