30 



THE INTERFEROMETRY OF 



Furthermore, though ju was determined by the total reflectometer for each 

 of the two plates used, B, the Cauchy dispersion coefficient, had to be assumed 

 from similar results in my earlier work. Finally, the first plate ( = 0.736 

 cm.) was slightly wedge-shaped and some adjustment for coincidence of 

 spectra was needed. The second plate ( = 0.434 cm.) was optically nearly 

 plane parallel. One may therefore conclude from these details that 2y = z as 

 nearly as could be expected. 



The values of x, computed from 6 and e, however, certainly fall below z, 

 being about 6 per cent and 3 per cent short of it in the two cases, respectively; 

 or, again, x is 0.019 cm. and 0.014 (about 5 per cent) smaller than the mean 

 values observed for zy. This extra 5 per cent of path-difference can not be 

 an error of observation or of adjustment, but must be interpreted as the path- 

 difference added when the pencil shifts towards the edge of the prism (x) 

 instead of being stationary as in y. In case of inverted spectra, moreover 

 (next chapter), % is usually in excess of z, and the shift is the other way. The 

 deficiency in x, though not equally marked, is present in observations both 

 on the right and left sides of the prism P' . 



16. Breadth of efficient wave=f routs. Apparent uniformity of wave=trains. 

 Rotation of fringes. It follows from figure 17 that if M is displaced to M f , 

 over a distance e, the pencil b is displaced parallel to itself over 



5 = 2e sin 5/2 



where 5 = 90 6. The pencil c is then displaced parallel to itself over a 

 distance 



t = s tan (f>'/2 = s 



since = 49 45', 6/2 = 20 7', and therefore 5 = 20X0.344 = 0. 7^, nearly. 

 If the rotation of fringes is but 90, either 5 (or 5/2) is also the breadth of 

 the strips, or patches of like origin, which, when sliding over each other more 



17 



18 



or less, produce the fringes. This may be treated from a graphic point of 

 view as follows, a theory not being aimed at: 



In figure 18, let a and b be two patches of light of like color and origin at 

 the objective pp, figure 17, producing interferences at the focus F. Hence 

 the fringes will be arranged in the direction/, figure 18, at right angles to the 



