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LIV. The Double Suspension Mirror. By L. Southerns, 

 M.A., P.Sc, Assistant Lecturer in Physics in the Uni- 

 versity of Sheffield *. 



DURING the course of a series of experiments with a 

 very delicate balance, in which a modification of the 

 " double suspension mirror" method of observing deflexions 

 was used, it became necessary to consider in detail. the effect 

 of the suspended system on the sensitiveness of the balance. 

 As the method is capable of other applications, and especially 

 as the attachment employed affords a convenient means of 

 varying the sensitiveness through any required range, a 

 description may be of interest to other users of sensitive 

 instruments of the balance type. 



A small mirror a b is suspended by two fibres as indicated 

 in fig. 1 — the plane of the diagram being the vertical plane in 

 which the knife-edge of the balance lies — from Q, the end of 

 the balance-pointer, and P, a support capable of adjustment, 

 by means of a screw, in a horizontal line parallel to the 

 knife-edge. A second screw for adjusting P perpendicularly 

 to the plane of the figure is sometimes necessary. We shall 

 first consider the deflexions of the balance-beam, and in this 

 case the mirror will act merely as a weight causing tension 

 in the fibres. Afterwards deflexions of the mirror itself will 

 be considered, these of course being much greater than those 

 of the beam. In this latter case, but hardly in the former, it 

 is necessary to damp the vibrations of the mirror by means 

 of a vertical wire projecting downward from it, carrying at 

 its extremity a disk or set of vanes dipping into a small vessel 

 containing oil. The viscosity of the oil should be as small 

 as is consistent with effectiveness in stilling the vibrations. 

 A good deal probably depends on the design of this damping 

 arrangement, but the matter has not vet been investigated. 

 It is desirable for practical reasons to arrange the points P Q 

 at different levels; they can then be brought as nearly as 

 may be required into the same vertical line without danger 

 of actual contact of the fibres. The inclination of the fibres 

 to the vertical has an important bearing on the theory of the 

 method. 



We suppose then for the present that a b represents a 

 weight attached to the fibres, and giving rise in them to 



* Communicated by Dr. W. M. Hick?, F.R.S. 

 Phil. Mag. S. 6. Vol. 36. No. 216. Dec. 1918. 2 K 



