On the Orientation of the Slit in Interference Experiments. 553 



Peters'* involve an approximation equivalent to thus neglecting 

 s in comparison with n, and the values he obtains for these 

 nutations are therefore on the whole too small, that for the 

 fortnightly, which is the most important, being about jf- of 

 what it should be. 



Taking the more accurate expressions for 6, yjr in (20), 

 (21), it appears that the semiaxes of the ellipse in the case of 

 a rigid earth being calculated by Peters, with the above error 

 as I think, to be 0" "0885 perpendicular to and 0"'08 1 2 parallel 

 to the plane of the ecliptic, those for the massless shell (€=300) 

 would be, with the error corrected, l ;/ '87 perpendicular to 

 and 2 // *03 parallel to the plane of the ecliptic. 



If the mass of the shell be taken into account, as its thick- 

 ness gradually increases from zero the fortnightly nutation 

 o-oes through a series of changes somewhat similar to that in 

 the case of the half-yearly. 



Dublin, Sept 16, 1898. 



LIX. On the Orientation of the Slit in Interference 

 Experiments. By James Walker, M.A.f 



IN a paper published in the Phil. Mag. for November 1898 

 I considered the question of the admissible width of the 

 slit in interference experiments, as carried out with Fresnel's 

 mirrors, the biprism, and the divided lens, on the assumption 

 that the slit was in its most favourable position, that is, 

 parallel to the intersection of the mirrors or to the edge of the 

 biprism, or perpendicular to the plane through the principal 

 axes of the two halves of the lens. 



M. Fabry, in a general discussion on the visibility of Inter- 

 ference Fringes J, has given an expression for the visibility in 

 the case of a faulty orientation of the slit, and has shown that 

 the effect is the same as that of a slit in its most favourable 

 direction, having a wddth equal to the projection of the actual 

 slit on a plane perpendicular to that direction §. This result 

 assumes that the actual slit is so narrow that the visibility of 

 the fringes, when it is properly adjusted, may be taken as 

 unity, and also that it is not tilted either towards or away 

 from the interference apparatus. The effect of such a tilt 

 M. Fabry has indeed considered || from a different point of 



* " Numerus Constans Nutationia," Memoires de VAcademie Imperiale 

 des Sciences. St. Petersburg (1843). 



t Communicated by the Author. 



X Thesis for the Degree of Doctor of Science, published at Marseilles, 

 1892. 



§ Loc. cit. p. 87.' || Loc. cit, p. 33. 



