38 THE INTERFEROMETRY OF 



and from the preceding equations, finally, 



where d\ would be the wave-length breadth of the fringe, remembering that 

 the fringes themselves are homogeneous light. 

 In the grating used 



cr 6 cm. X = 6oXicr 6 cm. w=ioocm. 



or 



400 



This is but 1/200 of the distance, cf\ = 6Xio" 8 cm., between the D lines. 

 Hence such fringes would be invisible. Moreover, ddcci/N; the fringes, there- 

 fore, should grow markedly in size as N is made smaller. Experiments were 

 carried out with this consideration in view, by the single-grating and concave- 

 mirror method, N being reduced from nearly 2 meters to 20 cm., without any 

 observable change in the breadth or character of the phenomenon. It showed 

 the same alternation of one black and one or two bright linear fringes, or 

 the reverse, throughout. Hence, it seems improbable that the phenomenon, 

 i.e., the interference fringes, are referable to such a plan of interference as 

 is given in figure 24. 



Similarly, for the case of two gratings, figure 23, 



cos frr/a) (Z/2 



2D'(N cos 



where, if we insert the data 0i = g4o' and 2 = i955' 



Z/=i73Xio- 6 cm. JV=i62cm. n = &2 cm. X = s8.9Xio- 6 cm. 



then 



Hfi _ .967XlO- 10 X26 3 I*-" radian* 



de ~ io-*X346X(i6 2 X.8 7 + 8 2 ) -33Xio radians. 



Thus dd is about of the same small order of values as above, i.e., less than 

 one-tenth second of arc or i/iooo of the DiD 2 space, and thus quite inappre- 

 ciable. Some other source, or at least some compensation, must therefore 

 be found for the interferometer interferences seen with homogeneous light. 



The full discussion of the effective path-difference in terms of the diffrac- 

 tions occurring will be given in 27, in order not to interrupt the progress 

 of the experimental work here. It will then be obvious that the mere effect 

 of changing the obliquity of the incident homogeneous rays, I, introduces 

 no path-difference, or that the fringes observed are varied by the displace- 

 ments and rotations of the grating, G', and the mirrors M and N. 



16. Experiments continued. Analogies. With this possible case disposed 

 of, it now becomes necessary to inquire into the other causes of the phenome- 

 non, as described in paragraph 13. This is conveniently done with reference 

 to figure 26, where n and n' are the axes of the pencil of yellow light, reflected 

 from the opaque mirrors M and N, after arrival from the transmitting grating 



