58 



THE INTERFEROMETRY OF 



corresponding displacements e, e', normal to G and G' and y in the direction 

 bb', may be registered. 



The adjustments, if symmetry were demanded, would be cumbersome; for, 

 in addition to precise modification of the position and orientation of the prisms 

 P, P', the grating requires fine adjustment and a means of securing parallelism 

 of the rulings. But an approximate adjustment does very well and no pains 

 were taken in the first experiments to secure symmetry. The spectra were 

 intensely brilliant in the low-order work; but even in the fourth order the 

 light was adequate. One may note that here the gratings do not reverse the 

 dispersion of the prism P, though this is relatively small. Table 12 is an 

 example of results : 



TABLE 12. Ranges 



gratings and 



of displacement varying with dispersion. Paired 

 60 prism. ^ = 46. 8=44. x 2ecosS/2. 



The fringes in the zero order were good and strong, not inferior to any of 

 the others, but unfortunately too short-lived. In the fourth order the fringes 

 are weak (although the enormous sodium doublets stand out clearly) , doubt- 

 less from excess of extraneous light. Here also it is difficult to prevent the 

 beam from vanishing at the edge of the prism P'. Hence the anomalously 

 small displacement, a discrepancy which is already quite manifest in the 

 third order. 



The present experiments furnish a striking example of the uniform breadth 

 of the strip of spectrum carrying the fringes, quite apart from the dispersion 

 of the spectra. In the prism spectrum, where the sodium doublets are indi- 

 cated by a hair-line just visible, to the fourth-order spectra, where they stand 

 apart like ropes, the linear phenomenon has the same width. 



The computation of the dispersive power in these cases is peculiar. It will 

 be seen from figure 38 that the angle 5 = 44 between the incident ray a and 



