66 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



well as by the micrometric shifting of the interferometer mirrors N r or M r . 

 There are thus two independent methods, apart from the ocular reading 

 of displaced fringes. 



The fringes from mm and nn may be made parallel by rotating nn on a 

 horizontal axis. If they are visible for the same micrometer position, the glass 

 paths must be rigorously the same in the two cases, so that optic plate is 

 needed. To make the fringes of the same size a vertical axis at nn suffices. 



58. Lens train. This would be peculiarly difficult to apply in the present 

 case. As a large field of view is desirable, a collimator will be needed. If F is 

 its external focus, supposed to be on m (fig. 92 vertical plane, 93 plan), the 

 lenses L can not be of very different focal power from the collimator, if too 

 much light is not to be lost at the edges. If f l and fa are the focal distances, 

 the total distances apart of the mirror m and n would not much exceed 2^-f 

 2 fa. It would probably not be advantageous to make this exceed 5 meters 

 and lenses of large diameter would be needed. The main difficulty, however, 

 is the introduction of one of these trains into each of the component beams, in 

 particular as it implies the insertion of uncertain glass paths for the rays. 

 In fact, though I obtained very good slit-images from each beam, the result 

 for two beams conjointly was inadequate. 



59. Estimate. It is finally of interest to ascertain the sensitiveness of an 

 arrangement like figure 91. If the distance 5 of mm from nn is passed in A* 

 seconds, and v is the velocity of light, 28 = v A* and A*/ 7= A0/27r,if the angu- 

 lar displacement A0 corresponds to A* and T is the period of the mirror. Fur- 

 thermore, on the interferometer A0 = A/V cos t/b, where b is the breadth of the 

 ray parallelogram, i the angle of incidence at the mirrors and AJV the microm- 

 eter displacement (normal to mirror) which corresponds to M. Hence 



cos 



If 5 =500 cm., 6 = 10 cm., T = o.i second, AN" = 10 4 cm., 



io J X6. 3 Xio 

 u= =9Xio 9 cm. 



T r>~ > y Tr\~ ' y r> T 



= 45, 



nearly. Since AAf = io" 4 cm. corresponds to about 2.5 fringes, v = 2.3 Xio 10 

 cm. /sec. per fringe. Now both 5 and 6 may be increased and T decreased, so 

 that the apparatus could easily be made more sensitive; nevertheless, the 

 present experiment is itself so extremely difficult and unpromising that I did 

 not further persevere with it. 



