80 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



they are better. Thus between D and E the agreement is within 7 X 10 4 cm. 

 It is obvious that between b and F different lines were sighted, some of them 

 possibly due to superimposed direct spectra. Thus for 



io 6 X = 5o. 411 = 49.58 &/V c = 0.02 13 5 = 0.03 591 fi tc. 



Both these and other lines seem to have been used. 



46. Summary. The results contained in tables 9, 10, n, reproduced in 

 fig. 43, show that equation (i) above, or any of its derivatives (4) and (8), 

 gives an accurate account of the motion of the center of ellipses throughout 

 the spectrum, even in case of such extreme conditions as are introduced by 

 glass columns 10 inches or more long. The constants of Cauchy's or any simi- 

 lar dispersion equation may therefore be obtained directly from observations 

 of this character. In such a case a linear interferometer, i.e., one in which 

 I = R = o approximately, would be specially convenient. If 8 refers to the 

 difference of the variables for 2 lines X and X' 



which in case of the simple dispersion equation gives ^B(E-\-e)d-^. As this 



linear interferometer will have other interesting properties, it has been thought 

 worth while to construct it in connection with the present work. 



49 50 



54- 56 58 60 6 64- 

 FIG. 43. 



The expectation of reaching great sensitiveness by using long columns was 

 not fulfilled in view of equation (17), which shows that the ellipses become 

 more and more sluggish in their motion through the spectrum, as the column 

 is longer. 



