THE AID OF THE ACHROMATIC FRINGES. 



47 



The constants of the apparatus were 



6 = 20 cm. ,8=71. 3 



Thus 



7 = 34-9 

 0.57 2) = 7. 5 



To obtain the ellipses a thick plate-glass compensator may be placed in 

 the d ray to provide for the elongation 2% in d' '. About 14 cm. of glass col- 

 umn were necessary. This makes it very easy to center the ellipses and to 

 obtain them intensely black on a colored ground by rotating the compen- 

 sator on a horizontal and vertical axis until the two strips of illumination at 

 p quite coincide when the rays T, T' are parallel. But on the other hand, 

 because of the thickness of glass used, the small ellipses obtained move rel- 

 atively sluggishly with displacement of the micrometers. The sensitiveness 

 decreases proportionately to the thickness of glass path. 



Experiments, of which the following data are examples, were made by 

 alternately restoring the center of ellipses to the D lines of the solar spectrum 

 first by the micrometer (A/V) and thereafter by the rotation of rail (Aa). 

 The adjustments were very different. 



The second series changed its rate enormously, almost one-half, owing 

 to necessary intermediate adjustments (inserting a new zero). Otherwise 

 the observations are as good as the apparatus permitted; but the computed 

 2 AA/YAa = 7.5 is above the observed value, usually, possibly owing to an ec- 

 centric position of the plate Pp relative to the axis of rotation. To test this 

 point of view the plate Pp was displaced eccentrically toward the left of the 

 axis. The result should be a modified coefficient, but the following data 

 obtained in the same way as before fail to bear this out, however: 



AAfXio 3 = o. 



19.9 

 5.9 



40.2 



12.2 



56.8 cm. 

 16.7 rad. 



The rate, 2AA/"/Ao: = 6.6, does not differ essentially from the above. With 

 these small ellipses there can not, of course, be an achromatic phenomenon. 

 To obtain large ellipses the glass-path difference (i. e., the dispersion) must be 

 abolished on both sides and an air-path difference introduced, preferably 

 in a way which has been shown above in figure 21. As such experiments 

 are so mach more trustworthy and sensitive, I did not pursue the glass- 

 column work further. 



