68 



THE INTERFEROMETRY OF 



do this effectively it was necessary to do the work at long distances (meters), 

 in order that adequate space might be available between opaque mirrors 

 and prism. Accordingly M, N, P', figure 43, were all placed on micrometers, 

 with the screws normal to the faces of the mirrors and the right edge of the 

 prism, respectively. The fringes were found without difficulty and they 

 were large and perfect near the edge of contact of the spectra. Though the 

 sunlight was waning, a few measurements of ranges of displacement were 

 made. They were on the average (e at M and N, y at P) : 



y = 0.062 cm. 



= o.og$ cm. 0^ = 0.097 cm - 



and since x=2ecos d/2 should correspond to 2y, 



cm. ## = 0.148 cm. 2y = o. 124 cm. 



Here, as above, x>2y, or the sliding along the edge of P which accompanies 

 e is distinctly effective, being nearly 16 per cent of 2y. 



Next day, with a bright sun, so that much finer fringes could still be de- 

 tected, the range could be increased to e M = o.2 cm. or #M = -3 cm. when the 

 spectra were all but separated on their near edges and fringes very large. 

 For the case of much overlapping of spectra, e M =o.i5 cm., x M = o.22 cm., 

 were obtained. Finally, when the spectra were all but separated on the far 

 edges (implying reflection at some distance from the edge of the prism P), 

 the fringes were glittering, but too small to be distinctly seen. 



TABLE 14. Inverted spectra. Long distances. Plate. = 0.434 cm.; n = 1.533; 



In table 14 the results for CM , VN, and y are given, when these displacements 

 are produced by a glass plate = 0.434 cm. thick. If the coefficient of dis- 

 persion B is assumed, the displacement computed from X, n, E would be 

 =0.243 cm. Although the values of zy are larger than this, the difference 



