REVERSED AND NON-REVERSED SPECTRA. 115 



Thus the vertical displacement of rays corresponding to the vertical semi- 

 axes of the central ellipse or one fringe is between 0.0065 an d 0.0067 cm. 

 i.e., on the average below 7 X io~ 3 cm. Hence A = TV X 0.007 for N such central 

 fringes. It was difficult to get a closer result, owing to quiver. 



The interesting question is now suggested, in how far such an arrangement 

 would fall short of being able to exhibit the drag of the ether in a rapidly 

 rotating body, should such drag occur. In figure 73, let a/3 be a cylinder of 

 glass with plane-parallel ends, capable of rotating on the axle 75. If / is the 

 length of the cylinder, /* its index of refraction, and r the distance of either 

 component ray (ac, bd) from the axis 75, n the number of turns per second, 

 and V the velocity of light, we may write, using the above excessive estimate, 

 N being the number of fringes displaced, 



h = ^2^X0.007 =2irnrliJ,/V 

 since ac rises while bd falls. If 



w = 2oo, r=iocm., /=ioocm., F = 3Xio 10 , ju=i.5, 



.. 6.3X2Xio 2 XioXio 2 Xi-5 



N = -=0.18, nearly 



3.5Xio- 3 X3Xio"> 



It would thus be necessary to estimate about one-sixtieth of a fringe, which 

 is just beyond the limit of certainty, even if nr can be increased and / multi- 

 plied by reflection. The device suggested is nevertheless of interest and 

 deserves further consideration. It will appear much more promising in 

 connection with the achromatic fringes described below. 



62. Displacement interferometer. Jamin type. These considerations 

 induced me to devote further study to the Jamin type of interferometer 

 (fig. 73). The mirrors M, N' were put on one pair of long slides (1.5 meters 

 long) parallel to ac and the mirrors M', N on similar slides parallel to the 

 former. In this way any distance ac or bd was available. The beams were 

 about 1 6 cm. apart, corresponding to a normal distance between the end 

 mirrors (A/TV', MM') of about 12 cm. But these distances could also be 

 increased from nearly zero (M and M' nearly contiguous) to about 20 cm. 

 in view of the width of mirrors used. The angles at a, b, c, d were each about 

 45, so that a rectangle of rays is in question. (See figure 88 or 93 below.) 



The adjustment proved eventually to be greatly facilitated by using a 

 horizontal beam of sunlight with weak condenser-lens and collimator. A thin 

 wire is to be drawn across the slit. M and M' are first set for parallelism in 

 the absence of N and N', by adjusting the images of the slit at the same level 

 (horizontal) on a distant wall. The images or shadows of the wire specified 

 on the wall are to be equally far apart, with the beams ac and bd at the mirror. 

 The mirrors N and N' are next put in place with the distances acd and abd 

 about equal. The two images seen in the telescope at T (g removed) are then 

 made to coincide both horizontally and vertically by adjusting N and N r , 



