116 



THE INTERFEROMETRY OF 



and these are then slid by a small amount on their slides (direction ac) until 

 the rays are coincident at d to the eye (light strips on the mirror coincide) . 



If, now, the grating g is inserted, very fine oblique fringes will usually be 

 seen. These may be enlarged to a maximum by moving the micrometer 

 controlling the displacement M' normal to itself. Somewhat coarser horizontal 

 lines are thus obtained. 



Finally, the distant centers of the ellipses are brought into the center of 

 the telescope by aid of the thick glass compensator, like figure 77 (the equiva- 

 lent air-path of the other ray being correspondingly lengthened) by rotating 

 the glass plate on a horizontal axis.* It is desirable to have an excess of glass- 

 path in one beam, as otherwise the ellipses are so large as to be unwieldy. 



The ellipses so obtained with common plate-glass and a film grating at g 

 were magnificent. A rough test of the displacement interferometer was made 

 by using the above plate-glass of thickness = 0.434 cm., where z=E(n i) 

 + 2.B/X 2 = o.2428 crn. In two experiments agreeing to within icr 4 cm., 20 = 

 0.3448 cm. were the displacements obtained. Assuming that = 45, 26 cos 

 = 0.2438 cm. This agrees with z as nearly as may be expected, unless 6 is 

 specifically measured. 



Experiments were now made (as above) with thick plate-glass compen- 

 sators inserted in one component ray (bd) only, to determine the rotation of 

 compensator (i) necessary to raise the center of ellipses in steps of half the 

 diameter of the first ring (see a, b, c, fig. 78). The initial angle i is already 

 large and shows the rotation of compensator from the vertical needed to 

 bring the ellipses into the field. Two sets of experiments were made with 

 plates respectively d = 0.965 cm. and d = 0.705 cm. in thickness, with the 



results given in table 30. 



TABLE 30. 



The equation of the preceding section is used for h. The first case shows 

 about the same order of sensitiveness (A/z. per half -ring). In the second case, 

 for the thinner plate, the sensitiveness has been more than doubled. This 



* The same result may be obtained in the absence of the compensator by rotating N and 

 N 1 on a horizontal axis, successively by small amounts, into parallelism with M and M'. 



