82 



THE INTERFEROMETRY OF 



To determine D from table 12 we have D=D'/n=i/(cos n -d 

 where n is the order of the spectrum and dd n /d\ its dispersive power, like n 

 is given in the table. In this way the data of table 15 were obtained. 



TABLE 15. Ratio of the range of displacement e observed in table 13 and 



de/dD, computed. 



In view of the character of the results for e, the ratio e/\/\/(D- J r\} z ) where 

 e is the observed range of displacement, may be considered constant. The 

 enormous variation of the range e with the dispersive power, as observed, must 

 therefore be regarded as in keeping with the theory of the phenomenon, 

 although the computation is not direct. The latter would require an integra- 

 tion of equation (22) which may be written 



but the simpler comparison given was regarded adequate. 



Data bearing on equation (20) are given in table n and may after reduc- 

 tion be written as in table 16 (0 2 = 19 30', D 2 = 177 X io" 6 ). 



TABLE 16. 



The value of the term in the last column is thus small in comparison with 

 2 and may be neglected, as a first approximation. Hence roughly 



and the range of displacement should be nearly independent of the dispersion. 

 As it is not, some corresponding principle must here be active, and this has 

 already been found in the shift of one illuminated strip on the collecting grating 

 relative to the other, when either of the opaque mirrors is displaced. 



For the same reason the effect produced by making 8 = 0, as in 24, is not 

 marked, so far as equation (20) is concerned. If 5 = o rigorously, the original 

 experiment with but a single grating did in fact show large ranges of displace- 

 ment, in view of the absence of sliding. 



We thus return to the special diffraction already mentioned in 38. When 

 two spectra from the same source coincide, horizontally and vertically, 

 throughout their extent, they will interfere at every point. The interference 



