112 THE INTERFEROMETRY OF 



The ellipses obtained are usually long vertically i.e., quite eccentric 

 so that the fringes soon become straight and the rotation is extremely rapid 

 whenever the center of ellipses is out of the field. It is therefore possible to 

 adjust relative to horizontal fringes (parallel to the shadow of wire across 

 slit), as these incline very obviously for a displacement of less than icr* cm. 

 and rapidly become vertical. For this reason it makes little difference in 

 practice whether the half-silvers are on the 

 same or on opposite sides, or whether obser- 

 vation be made at T (cd prolonged) or at T' 

 (bd prolonged). Moreover, the plate MN 

 may be conveniently constructed, as in figure m Q c | _ ^ 



75, of two mirrors m, n, attached to the clear I - 1^ 



strip of plate-glass g by aid of strong steel 76 



clips at c, c'. With the half-silvers 5, s', on the same side, the wedge-angle 

 of the glass is excluded. For shorter diagonals, the plan of figure 76, with 

 the silver surfaces s, s' held together by clips at c, is preferable. 



If the mirror M', figure 74, is displaced a distance e, where a glass-plate 

 compensator of thickness E and refraction constants /z and B is introduced 

 normally either into ab or bd, the equation is easily seen to be, at wave- 

 length X, 



6 



where 6 is the angle of reflection at M. Using the plate = 0.434 cm. treated 

 above, the first member is 0.2428 cm. Values of e of 0.2420, 0.2409, 0.2427 

 were roughly obtained. Hence the mean value of 6 should be about 60, 

 as it actually was. 



The occurrence of this angle and the shift of the beam bd along the mirrors 

 M' and N are the main objections to the method of figure 74, for the rhom- 

 bus is not necessarily perfect. If the ends of the plate MN are silvered on 

 the same side, the compensator must have double the plate-thickness to 

 annul path-difference. Finally, the half -silvering does not, for large 6, suffi- 

 ciently exclude the reflection of bd from the naked face of the plate, so that 

 the fringes are never quite black. These difficulties may be met by making 

 MN, figure 74, the short diagonal of the rhombus and using the strip, figure 

 76. In such a case 6 at M' is small, and in view of the nearly normal reflec- 

 tion at M and N relatively little reflection comes from naked glass, sliding 

 is largely avoided, and no compensator is necessary. In this case the fringes 

 for no path-difference are actually strong black horizontal lines on a colored 

 ground and far enough apart that o.i fringe could easily be estimated. A 

 test experiment with the above plate showed = 0.1244 cm., corresponding 

 to the small angle 9, a little over 12. 



When the U-tube C C', figures 71 and 74, is introduced, the strip MN will 

 have to be at a considerable angle (about 45) to the horizontal, so as to 

 raise the N end about 15 cm. above the M end, corresponding to the height 

 of m above m' in figure 71. The new condition, however, in no way changes 



