88 THE INTERFEROMETRY OF 



and 5' are the traces of two longitudinally coincident reversed spectra, drawn 

 apart for distinction, the region of the D lines only being used. The light is 

 homogeneous to this extent and the slit wide, so that there is oblique inci- 

 dence. Then every point of S should (on adjustment) interfere with every 

 point of 5', the result showing a uniformly striated field in the telescope. 

 This is emphatically the case for the parallel rays, mM, nN; but with the 

 crossed rays, mN, nM, the interference is confined to the rays in the equi- 

 distant positions, n, in figure 63, and midway between them the field is a 

 neutral yellow. In other words, between the rays n the rays are displaced, 

 as shown by the arrows, recalling the arrangement of nodes in acoustics. 



Corresponding rays a and a' (for instance) do not coincide and hence can 

 not interfere, the region aa' remaining neutral. In figure 64 the rays crossing 

 at c (fig. 57) have been shown for three nodes and the transverse arrows indi- 

 cate the directions in which the rays have been urged laterally. Naturally, I 

 am merely stating the case as immediately suggested by the results. One may 

 argue that there may be a secondary periodicity in the grating. But why 



does it not appear at all in the case of parallel pencils, when it is so obtrusive in 

 the case of crossed pencils of rays ? Again, the interferences are unquestion- 

 ably due to Di and D 2 light, simultaneously. If the grids for these two wave- 

 lengths should be at a slightly different angle to each other, their superposition 

 would give something like the observed phenomenon, apart from details. Thus 

 in figure 65 the two grids due to DI and D 2 , intersecting at a small angle, may 

 be interpreted as appearing strand or cord like at N, and neutral at I and I'. 

 With white light the linear phenomenon would eventually become achromatic. 

 But, again, why should lines so close together as DI and D 2 show any appre- 

 ciable difference of angle or rotational phase-difference in their interference 

 pattern? Intersecting grids, moreover, can be produced by other methods 

 and nearly always betray their origin. The final inference is that suggested 

 by figures 63 and 64, that homogeneous rays on crossing (here in a medium 

 of plate glass) may exert a lateral influence on each other, to the effect that 

 identical rays emerging from the crossing are arranged in equidistant nodal 

 planes according to figure 63. 



40. Experiments. Reflecting grating. Crossed rays. In the preceding 

 experiments the remarkable phenomenon of double interferences was ob- 

 tained with glass-plate apparatus. It is improbable that any secondary inter- 

 ference can have been produced by the presence of reflected light, since the 

 reflected pencils will be weak as compared with the primary pencils and 



