62 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



The preliminary adjustment consists in raising or lowering the vise v on s, 

 until the spring s vibrates naturally in step with the interrupter . When this is 

 sufficiently nearly the case, the soft-iron core 55 of the electromagnet M is 

 first pushed towards the springs and then gradually withdrawn from it, 

 until the amplitude of L (using it in connection with the remainder of the 

 telescope) is a maximum. For this purpose, the core 55 is a fine-threaded 

 screw, snugly fitting a fixed socket within M. Thus the fine adjustment 

 for a maximum amplitude of L or maximum width of fringe band may 

 be made to a nicety up to the point at which the vibration becomes 

 unstable and s drops back to its position of equilibrium. Amplitudes of 

 i cm. at L are easily obtained, unless the spring s is too short. Care 

 must be taken to prevent an excess of spring, 5', from interfering with the 

 vibration of s. 



55. Shattered fringes. When the fringe- waves are observed with the syn- 

 chronized electromagnetic apparatus just described, their appearance in general 

 may be indicated as in figure 89. There are regions x, x f at the two ends, 55', 

 of the fringe band, consisting of shattered waves (as I may call them) and the 

 impression produced on the observer is that of fringe rays radiating from a 



center. Frequently, x' is absent and the phenomenon is confined to one side. 

 Following x are a series of wave-lines w, of decreasing amplitude and increasing 

 length, the form of which, however, is not sinusoidal, but rather like a succession 

 of cycloids (either erect or inverted) , or very much like a succession of festoons. 

 They are nearly stationary. When the telephone current is weak (20,000 ohms 

 in circuit), the region x all but vanishes and the waves begin at the edge s 

 of the fringe band and extend toward the middle m. When the current is 

 increased, x rapidly increases and the waves w are driven toward the middle 

 and ultimately (if x' is absent) across the field to the opposite edge s'. 



If X is the observed wave-length at the center of the field T, and a the period 

 and amplitude of the vibration telescope, the period of the shock- waves is 

 easily seen to be t = \T /ma. The ocular micrometer showed X = 5 and a= 100 

 scale-parts, while T = o.i second, so that * = io~ 4 X8 seconds. Owing to the 

 quiver, \ is hard to determine; but the waves affecting the interferometer 

 in such marked degree occur with a frequency of the order of r,oooper second 



