REVERSED AND NON-REVERSED SPECTRA. 



119 



The equations are liable to be cumbersome in the cases of greatest interest. 

 I therefore proceeded experimentally to obtain a limit of Ah or Ax per fringe, 

 using thinner glass compensators. The results are given in table 33. The 

 ellipses were now so large and distorted that it was difficult to define the 

 center of irregular rings. The transverse displacement is therefore largely 

 referred to the top, middle and bottom of the spectrum band, which took 

 up about one-third of the height or diameter of the field of the telescope. 

 The angular width of the latter being about 3, the corresponding angular height 

 of the spectrum is thus about 1.0. In view of the large rings, moreover, the 

 displacement AN at the micrometer of the mirror M' is difficult to obtain and 

 the data given are estimates. Experiments like the present must be made 

 with optic plate-glass, so that sharp rings nearly circular may be obtained, 

 if the data are to be quite satisfactory. In table 33, Ah thus corresponds to 

 a transverse displacement of one component ray parallel to itself, equivalent 

 to a displacement of the centers of ellipses of about 0.5 at the sodium line. 



TABLE 33. Vertical (transverse) displacement of ellipses. Glass-plate 

 compensators M = 1-53- Horizontal axis. Angle of telescopic field 3; 

 angular height of spectrum about 1.0 or 0.175 radian. Vertical 

 diameter of first fringe in excess of height of spectrum. 



The mean result of all data is here about Ah = 0.002 cm., and this is not 

 influenced in a discernible way, either by the thickness of plate e or by the 

 rotation angle of the compensator i. The smallest value Ah = 0.0012 cm. 

 appears incidentally and not when the system of four mirrors is most nearly 

 in parallel adjustment. The transverse displacement of ellipses changes sign 

 with the sign of the rotation of i in all cases and is independent of the normal 

 position. The longitudinal displacement reverses at the normal position. 



