58 



DISPLACEMENT INTERFEROMETRY BY 



to about the same period. The electromagnet e' could be rotated on a vertical 

 axis, so that by moving it closer to or farther from the steel spring of V any 

 degree of band-width was obtainable in the telescope. Magnificent octave 

 fringes (d, figure 60) were obtained in this way. They moved merely on 

 opening and closing the circuit. Otherwise either the stationary type a or d 

 permanently occupied the field. The fringes were still small, but a double 

 amplitude of about 5 scale-parts was registered as before. Again, by changing 

 the tension of the wire, one may pass from a through d back to a again. 



The fundamental has too small an amplitude to be striking, but the second 

 overtone develops well. The form e is producible, but what usually appears 

 is apt to resemble/. Changes of tension may be used to generate the stationary 

 figure. 



Fringes which have vanished (from slight disadjustment) may frequently 

 be recovered by changing the focus of the telescope. In fact, care must be 

 exercised against the possibility of vibration of other parts of the system, the 

 telescope for instance. 



42. Decreased bifilar distances. The easy accomplishment of the above 

 experiments, where the distance between the bifilar threads was about 30 cm. 

 and their length 60 cm., encouraged me to reduce the distances between the 

 threads until the system was virtually torsional. 

 In such a case the displacement at the ends of the 

 vibrating beam cc', figure 61, is no longer limited 

 to that of the bifilar wires. Figure 61 is a front 

 elevation of the new apparatus, the steel-wires dd 

 being at a distance of about 6 cm., their length 60 

 cm., the distance between the auxiliary mirrors 

 m,m' 10 cm., and between the magnets of the tele- 

 phones T, T', 1 6 cm. The vibrator cc' was (as 

 above) a steel umbrella rib. The ends of the wires 

 dd' were again wound about the stiff screws b, b' 

 held in posts a, a' rigidly attached to the bed-plate 

 of the interferometer (not shown) . The wires were 

 stretched below by the pulley w and screw y, con- 

 trolled by the nut u pressing against the rail v, also 

 rigidly attached to the bed-plate of the interfer- 

 ometer. Provision must be made (slotted sheath z and pin p) to prevent y 

 from turning on its axis. One of the telephones is to be adjustable on a screw- 

 spring device (not shown) to regulate its distance from the vibrator cc' to 

 correspond with the other telephone. Again, a variety of braces are to be 

 introduced to obviate synchronous vibrations of parts of the apparatus 

 (especially of the telephones), in so far as needful. 



The adjustment for resonance is here more difficult than in the preceding 

 case, and y must be a fine or a differential screw and the nut u work smoothly. 

 Resonance should first be established by aid of the slit-image in a telescope 



FIG. 61. 



