56 DISPLACEMENT INTERFEROMETRY BY 



The vibrator proper, cc' , is attached at the middle of the wires d, d' and 

 carries the parallel auxiliary mirrors m, m' of the quadratic interferometer. 

 A thin steel umbrella-rib seemed well adapted to fulfill the requirements of 

 cc', though a light soft-iron tube would have been preferable. 



The telephones T and T are adjustable on special standards, attached to 

 the bed-plate (carriage BB) and placed horizontally, one in front and the 

 other toward the rear of the vibrator cc'. It is desirable that one be adjustable 

 on a micrometer screw and spring, so that the distance of the poles of both 

 from cc' may be regulated to correspond with each other and the tension of 

 the wire, as explained in the preceding paper. 



The achromatic fringes in the fine slit-image of the telescope field of the 

 interferometer must be observed with a vibration telescope, and it was found 

 desirable to control the latter by a special electromagnet. Figure 59 is a 

 diagram of the parts of the apparatus as a whole, M, M', N, N' being the mir- 

 rors of the interferometer (M' on a micrometer-screw s) ; m, m' the auxiliary 

 mirrors on the vibrator cc'; T, T', are the telephones (one provided with a 

 switch r), V the vibration telescope, / the mercury interrupter. T" is an 

 auxiliary telephone for the ear. 



Thus the primary consists of the linear coil P described in the preceding 

 paper, the storage-cells E (usually four), and the two small electro-magnets 

 e, e', for controlling the interrupter and the objective of the vibration tele- 

 scope V. The secondary 5 was the coil "B " of the preceding paper, wound 

 on glass. This was in circuit with a rheostat R (up to 40,000 ohms), the tele- 

 phones T, T', T", and the key U. The condenser C (up to 4 microfarads) is 

 available when needed. 



The whole of the parts shown in figure 58 could be slid fore and aft on the 

 carriage BB and slides A, A' to accommodate the interferometer. Inductive 

 resistances e" were later added to the primary and the key K was replaced 

 by a switch or commutator. 



40. Observations with the slit=image. It will be seen that if an ordinary 

 telescope is used at V, figure 59, the sharp slit-image must widen to a band 

 here, as in the preceding apparatus; but it is much less sensitive because the 

 rays are parallel throughout. It nevertheless suffices admirably for finding 

 the resonance tension. For this purpose the screw b or b', figure 58, is first 

 manipulated till the image begins to widen. The fine adjustment is then 

 made at u, y, till the maximum band- width is reached. One easily recognizes 

 in this way three harmonics the fundamental at lowest tension and small 

 band-width, the octave at larger tension with maximum band-width, and the 

 next overtone at still larger tension and diminished band- width. Above this 

 I did not go, as the stress on the wire would have been excessive. The reason 

 for the prominence of the octave here is not clear to me. It does not occur 

 again in the later work. While the fundamental showed a band width of 

 but 5 scale-parts, the same for the octave was 42 scale-parts wide; and for 

 the next harmonic it was 26 scale-parts wide. 



