104 



THE INTERFEROMETRY OF 



same trough but with different solutions, dilute and nearly concentrated. 

 To place the trough normal to the beam, the reflection of the latter from 

 the face of the trough was made to coincide with the spot on the half -silver. 

 This is inadequate for fine work, but the interferometer method of retro- 

 gressing fringes is not applicable unless the trough is separately mounted. 

 From the data obtained the constant B was computed from pairs of Fraun- 

 hofer lines, B and E, C and b, D and F. Apart from the effect of the thick- 

 ness e, it should be correct in case of the solutions to about a few tenths per 

 cent. In case of the concentrated solutions, however, the ellipses already 

 begin to move sluggishly and are much smaller. 



48 50 52 54 56 58 60 6& 64 66 68 70 



From ju I + 2.5/X 2 and B, the corresponding indices of refraction in table 27 

 are easily computed; but these data are of little value here, because the thick- 

 ness e of the ordinary plate-glass used is not constant to the degree necessary. 

 It is, however, worth while to exhibit the B for the different mercury-iodide solu- 

 tions in terms of either /t or, what is equally serviceable and more convenient, 

 in terms of JLI i-f-K"; for this quantity is directly given by the displacement 

 measurement &N/e. If we regard the mean B for the B to E lines, and C to 

 b lines as applying to the D line, the coordinated values are: 



(2) (4) (8) (9) (10) 



H-i+K 0.5384 0.5869 0.8732 0.8736 0.6194 



JSXio 11 15-63 18.37 4-99 40.98 20.94 



For solutions of small concentration the ratio of B and pi+K changes 

 but slowly (29 to 34) as shown in figure 69, so that B may be predicted from 

 the latter, always remembering that our equation with two constants (n = A 

 -+-5/X 2 ) is inadequate at the outset. 



