546 Mr. G. A. Shakespear on an Interference-Method for 



cross-section shown in fig. 8. In the centre of each disk i^ a 

 hole, that in B 1 for the suspending bolt and that in D 5 for the 

 hook which supports the weight, In each slot slides a movable 

 piece -J inch long, and of section shown in fig. 9, with two holes 



Fig. 9. Fi<r. 10. 



through its length : the larger one admits the radial adjusting 

 screw ; into the smaller one an end of the wire to be tested 

 fits (bent at right angles), and is clamped there by a brass 

 clamp (<2, fig. 7) fitting over the free part of the sliding-piece. 

 The head (fig. 10) of the radial screw is squared to fit a clock- 

 key and has also a pulley-groove in it ; into this groove, when 

 the sliding-piece is in position, the hinged piece of flat brass, 

 shown in fig. 8, fits and is then secured by the small screw 

 shown on the other side of the slot. In this way the radial 

 screw, unable to move backwards or forwards, is able to move 

 the sliding-piece along the slot. 



Next below this comes the prism-suspending disk D 2 , with 

 three narrow radial slits just wide enough to admit No. 20 

 B.w.G. wire freely ; this is supported on the three upper 

 clips. Each clip is made of brass with a hemispherical head 

 of steel, the latter consisting of two sharp-edged jaws which 

 can be screwed together (two of these clips are shown in 

 position on wire in fig. 7, bb). 



Near each of these radial slits in D 2 is a small hole into 

 which a screw from the edge passes. From these holes pass 

 wires of the same material as that under investigation, to 

 support D 3 on which the prism-stand itself rests. D 3 has three 

 large notches in its edge, and through these rise, without 

 touching, the pillars from D 4 , which bear vanes (one of which 

 is shown in position in D 4 , c, fig. 7) to dip into three corre- 

 sponding dash-pots which stand on D 3 . Near the centre of D 3 

 are three large holes to admit, without contact, the three 

 levelling-pegs of D 4 , on which the mirror rests. The prism- 

 stand (/', fig. 7) carries the prism over the surface of the 

 mirror (g), and is provided with three le veiling-screws as feet 

 for coarse adjustment. D 4 is the mirror-plate with three radial 

 slits, it rests on three clips. It has the mirror supports, which 

 are provided with a fine adjustment consisting of three radially 



