94 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



In the case i -<- 4, the component rays pass through the grating, one and three 

 times, respectively. The corresponding path difference may be called 2 5. The 

 micrometer position of the mirror M was at the arbitrary mark 0.525 cm. 

 The interferences were very fine hair-lines, evidencing a very distant center. 

 They had the usual tendency of revolving from vertical to horizontal when 

 they are thickest, into vertical again, when M moves; i.e., enormous concentric 

 equidistant circles are moving through the field horizontally, the center being 

 invisible. The data correspond roughly to the horizontal at the sodium line. 



Case i + 5 showed the same phenomenon of fine revolving hair-lines, seen 

 to be circles when horizontal. Their micrometer position, however, was 1.275 

 cm., roughly, so that the displacement of M of 0.75 cm. has intervened. The 

 path difference is 5. 



Neither of these cases is of value unless the centers happen to be close at 

 hand. This is a matter of chance residing in the wedge shape of the plate. 



FIGS. 54 TO 59. 



55. Continued. The groups 2+4, 2+5. Both of these interference pat- 

 terns are interesting, as the lines are strong or centered ellipses. The nature 

 of the interferences is given in figs. 56 and 57 with the same notation. 



In case 2 + 4, both component rays pass through the glass twice. The path 

 difference is therefore 8 = 0, and the micrometer position of M, 2.025 cm -> an 

 advance of 0.75, as before. The system is self -compensated. The fine, very 

 strong, revolving lines, circular when horizontal, are reticulated (the center, 

 however, not being in the field), showing a second set of interferences not 

 nearly so strong, crossing the former set usually at about 45. The fainter 

 lines may be obtained by slightly revolving the mirror M about the vertical 

 axis by aid of the adjustment screws. These reticulated forms occur only for 

 the compensated adjustment. They are due to the two reflections possible 



